Development of Hydrologic Characterization Technology of ...

Development of Hydrologic Characterization Technology of Fault Zones

Kenzi Karasaki, Tiemi Onishi, and Yu-Shu Wu

Earth Sciences Division, Lawrence Berkeley National Laboratory

March 2008

NUMO-LBNL Collaborative Research Project Report

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This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof, or the Regents of the University of California.

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Executive Summary

This is the year-end report of the first year of the NUMO-LBNL collaborative project: Development of Hydrologic Characterization Technology of Fault Zones under NUMO-DOE/LBNL collaboration agreement, the task description of which can be found in the Appendix.

Literature survey is conducted to study past works that relate geology to hydrology of fault zones. This includes study in sediments, sedimentary and crystalline (igneous and metamorphic) rocks with examples from various parts of the world. We focus on studies of hydrology of fault zones in the field and the laboratory as well as through modeling performed in all types of faults (i.e. normal, thrust and reverse, and strike-slip). We find that there is very limited amount of work on the subject, particularly in the field using borehole testing. The common elements of a fault include a core, and damage zones. The core usually acts as a barrier to the flow across it, whereas the damage zone controls the flow either parallel to the strike or dip of a fault. In most of cases the damage zone is the one that is controlling the flow in the fault zone and the surroundings. The permeability of damage zone is in the range of two to three orders of magnitude higher than the protolith. The fault core can have permeability up to seven orders of magnitude lower than the damage zone. In general, most fault studies in the literature fit into combined-conduit barrier classification by Caine et al., 1996.

The main results are listed in three tables which summarize the main geological, structural and hydrological characteristics for each type of fault. At the end of the chapter, we suggest a characterization strategy for classifying faults based on available information (i.e. stress field, scaling relationship, fluid flow concentrated on one side of a fault) The fault types (normal, reverse, and strike-slip) by themselves do not appear to be a clear classifier of the hydrology of fault zones. However, there still remains a possibility that other additional geologic attributes and scaling relationships can be used to predict or bracket the range of hydrologic behavior of fault zones. We identify potential U.S. locations to conduct analogue studies and to extend the current effort on fault zone classification and development of characterization technology.

Next we examine in the literature the characterization technologies employed and the information used to identify the hydrologic properties of faults. The findings are listed by the location. AMT(Audio frequency Magneto Telluric) and seismic reflection techniques are often used to locate faults. In some cases AMT results are successfully used to infer the hydrologic properties. Geochemical signatures and temperature distributions are often used to identify flow domains and/or directions. ALSM(Airborne Laser Swath Mapping) or LIDAR (Light Detection and Ranging) method may prove to be a powerful tool for identifying lineaments in place of the traditional photogrammetry. Nonetheless not much work has been done to characterize the hydrologic properties of faults by directly testing them using pump tests. There are some uncertainties involved in analyzing pressure transients of pump tests: both low permeability and high permeability faults exhibit similar pressure responses.

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A physically based conceptual and numerical model is presented for simulating fluid and heat flow and solute transport through fractured fault zones using a multiple-continuum medium approach. The suggested multiple-continuum concept is a natural extension of the classic double-porosity model, with the fracture continuum responsible for conducting global flow, while vuggy (if any) and matrix continua, locally connected and interacting with globally connecting fractures, provide storage space for fluid and solute. The proposed conceptual model can be implemented into a general multidimensional numerical reservoir simulator TOUGH2 using a control-volume, finite-difference approach, which can be used to simulate single-phase flow, solute transport, and heat transfer in 1-D, 2-D, and 3-D fractured faulted systems. Model application is demonstrated by modeling two well-flow problems at ambient geothermal and water flow conditions. Data from the Horonobe URL site are analyzed to demonstrate the proposed approach and to examine the flow direction and magnitude on both sides of a suspected fault.

Finally, we describe a strategy for effective characterization of fault zone hydrology. We make a step-by-step recommendation of the technologies to be employed and decisions to be made along the preliminary characterization process. The recommendations are written in Japanese to ensure quick and easy consumption by the NUMO personnel. We recommend conducting a long term pump test followed by a long term buildup test. We do not recommend isolating the borehole into too many intervals. We do recommend to ensure durability and redundancy for long term monitoring.

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Table of Contents Executive Summary ........................................................................................................... iii Table of Contents ................................................................................................................ v List of Tables ................................................................................................................... viii List of Figures .................................................................................................................... ix 1 Introduction ................................................................................................................. 1 2 Geological Structure of Fault Zones and Hydrology .................................................. 4

2.1 Introduction ......................................................................................................... 4 2.2 Fault Anatomy .................................................................................................... 5 2.3 Structural characteristics of faults in various rock types .................................... 6

2.3.1 Sediments and Sedimentary rocks .............................................................. 6 2.3.2 Crystalline and volcanic rocks .................................................................... 8

2.4 Survey of past and current hydrologic characterization activities of fault zones........................................................................................................................... 10

2.4.1 Normal Faults ............................................................................................ 11 2.4.2 Thrust and Reverse Faults ......................................................................... 18 2.4.3 Strike-slip Faults ....................................................................................... 23

2.5 Scaling relationship between fault components ................................................ 28 2.6 Features to consider for hydraulic characterization of fault zones: .................. 32 2.7 Analogue studies in the USA ............................................................................ 33

2.7.1 Suggested target study areas in the USA .................................................. 33 2.7.1.1 California State ..................................................................................... 34 2.7.1.2 Colorado State (possible collaboration) ................................................ 39

2.8 Summary ........................................................................................................... 41 2.9 Characterization Strategy .................................................................................. 43 2.10 References ......................................................................................................... 45

3 断層の調査技術の実例、解析及びモデル化....................................................... 55 3.1 諸外国での事例............................................................................................... 55

3.1.1 アス、ブラジル....................................................................................... 55 3.1.2 アンデス南東域、ボリビア................................................................... 56 3.1.3 レッドシーヒルズ、エジプト............................................................... 57 3.1.4 北サマーセット、イギリス................................................................... 58 3.1.5 オランダ................................................................................................... 59 3.1.6 ベルギー................................................................................................... 62 3.1.7 ブリタニー、フランス........................................................................... 63 3.1.8 プロバンス、南東フランス................................................................... 63 3.1.9 コリントス湾、ギリシャ....................................................................... 64 3.1.10 コリントス、ギリシャ........................................................................... 65 3.1.11 Sobreda, 北ポルトガル ........................................................................... 66 3.1.12 タグスバレー、ポルトガル................................................................... 67 3.1.13 Lac du Bonnet, Canada ............................................................................. 68 3.1.14 ポンジョンギ、韓国............................................................................... 68 3.1.15 台湾........................................................................................................... 69 3.1.16 Fairbanks, Alaska ...................................................................................... 70

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3.1.17 Blue Ridge Province, Virginia .................................................................. 71 3.1.18 ルイジアナ州........................................................................................... 72 3.1.19 アルボード、オレゴン、米国............................................................... 74 3.1.20 ワシャキエ山脈、ワイオミング州....................................................... 75 3.1.21 ハンナ盆地、ワイオミング州............................................................... 76 3.1.22 リオグランデリフト、ニューメキシコ州........................................... 77 3.1.23 ロスアラモス、ニューメキシコ........................................................... 78 3.1.24 East Tintic Mountains ユタ州 .................................................................. 79 3.1.25 ハリケーン、ユタ州............................................................................... 80 3.1.26 Big Hole fault、ユタ州 ............................................................................ 81 3.1.27 エルクホーン、コロラド州................................................................... 82 3.1.28 Medina, テキサス .................................................................................... 83 3.1.29 Hidden Valley Fault, Texas ...................................................................... 84 3.1.30 Yuuca Mountain、ネバダ........................................................................ 86 3.1.31 Nevada ...................................................................................................... 91 3.1.32 Valley of Fire, Nevada .............................................................................. 92

3.2 我が国の事例................................................................................................... 94 3.2.1 超深地層研究所近辺............................................................................... 94 3.2.2 幌延深地層研究サイト........................................................................... 96

3.3 ALSM (Airborne Laser Swath Mapping) ......................................................... 97 3.4 断層水理の解析解と数値モデル................................................................... 98

3.4.1 断層のある井戸試験の解析解............................................................... 98 3.4.2 不均質な断層のシミュレーション....................................................... 99 3.4.3 断層を跨いだ水理試験......................................................................... 100

3.5 まとめ............................................................................................................. 102 4 Conceptualization and Modeling of Fault Zones .................................................... 109

4.1 Introduction ..................................................................................................... 109 4.2 Conceptual Model of Fractured Faults ........................................................... 111 4.3 Mathematical Model ....................................................................................... 115

4.3.1 Single-Phase Darcy Flow ........................................................................ 116 4.3.2 Mass Transport........................................................................................ 117 4.3.3 Heat Transfer .......................................................................................... 118 4.3.4 Constitutive Relationships ...................................................................... 119

4.4 Numerical Formulation and Solution .............................................................. 119 4.4.1 Discrete Equations .................................................................................. 119 4.4.2 Non-Darcy and Other complex Flow ...................................................... 123 4.4.3 Effect of Rock Deformation .................................................................... 124 4.4.4 Handling Fracture-Vug-Matrix Interaction in Faults .............................. 126

4.5 Numerical Solution Scheme ........................................................................... 128 4.6 Treatment of Initial and Boundary Conditions ............................................... 129 4.7 Simulation Example ........................................................................................ 130 4.8 Summary ......................................................................................................... 140 4.9 Recommendations for Future Work ................................................................ 140 4.10 References ....................................................................................................... 141

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5 概要調査地域に於ける断層の調査法に関して................................................. 145 5.1 調査のストラテジー..................................................................................... 145 5.2 断層調査のステップ..................................................................................... 146

5.2.1 文献調査................................................................................................. 146 5.2.2 衛星、空中測量によるリニアメントの抽出..................................... 146 5.2.3 地上物理探査......................................................................................... 147 5.2.4 地表地質調査とトレンチング............................................................. 147 5.2.5 ボアホールによる調査......................................................................... 148 5.2.6 長期モニタリング................................................................................. 150

5.3 フローチャート............................................................................................. 150 5.4 References ....................................................................................................... 152

Appendix 1 ...................................................................................................................... 154

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List of Tables

Table 1-1 Normal Faults ................................................................................................. 17

Table 1-2 Thrust/reverse faults ....................................................................................... 21

Table 1-3 Strike-slip faults ............................................................................................. 25

Table 4-1 Characteristic distances* for evaluating flow terms between fractures, vugs, and matrix systems ....................................................................................... 128

Table 4-2 Geologic units/layers and measured temperature and pressure data for Well HDB-7 .......................................................................................................... 132

Table 4-3 Geologic units/layers and measured temperature and pressure data for Well HDB-8 .......................................................................................................... 133

Table 4-4 Measured total mineral concentrations and calculated density factors used for modifying water density as a function of total concentrations at depth ....... 135

Table 4-5 Permeability and porosity values for different geologic units/layers .......... 135

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List of Figures

Figure 2-1. Deformation bands at Estrada Sandstone. Fossen and Bale, 2007 ................ 7

Figure 2-2 (a) Strike-Slip Punchbowl Fault (inactive strand of the San Andreas Fault) juxtaposing sandstone and granite; (b) photo showing ultracataclasite (from field guide to Punchbowl Fault, Chester, 1999) .......................................... 10

Figure 2-3 Permeability of host rock plotted against that of demormation bands…….14

Figure 2-4 Deformation bands at Estrada Sandstone. Fossen and Bale, 2007 .............. 15

Figure 2-5 Shows the distribution of permeability across fault zone from Evans, Forster, Goddard, 1997 ................................................................................ 19

Figure 2-6 Compilation of displacement-length data in 2D. DB=deformation bands (Schultz and Fossen and reference therein, 2002). ...................................... 30

Figure 2-7 Log-log displacement–length from Schilische et al. (1996). ....................... 30

Figure 2-8 Plot of process zone (fault thickness) width versus fault length for this study and additional published data (Brock and Engelder, 1977, Cheraysbev and Dearman, 1991, Little, 1995 (Jenssen et al., 2002) ..................................... 31

Figure 2-9 It shows the relationship between width of shear zone and displacement for strike-slip faults ........................................................................................... 32

Figure 2-10 Location of LBNL site (Google map) .......................................................... 34

Figure 2-11 Distribution of geological formation at LBNL site ...................................... 35

Figure 2-12 Geological cross section shown in Fig.2-11 ................................................ 36

Figure 2-13 Proposed locations for trenching .................................................................. 37

Figure 2-14 Location of Black Diamond Mine and distribution of main faults .............. 38

Figure 2-15. Picture showing the Hazel-Atlas fault in the Black Diamond Mine, Antioch, CA ................................................................................................................ 39

Figure 2-16. Location of Elkhorn Fault, Colorado ........................................................... 40

Figure 2-17 Geology and location of wells across the Elkhorn Fault (by http://www.agu.org/meetings/fm07/fm07-sessions/fm07_H23A.html ) ..... 41

Figure 3-1 a)ブラジルアス貯水池 b) 微小地震と水深の関連性 c)水位変動と地震

の遅れ（do Nascimento et al., 2005） ...................................................... 56

Figure 3-2 断層の部位によって異なる炭素と酸素の同位体の分布(Labaume et al, 2000) ............................................................................................................ 57

Figure 3-3 Red Sea Hills、Egypt の横ずれ断層と帯水層(Sultan et al., 2008) ......... 58

Figure 3-4 横ずれ断層沿いの不均質性の偏り b),f):割れ目頻度、c),g)開口幅、

d),h) コンダクタンス分布。Leckenby et al, (2005) ................................ 59

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Figure 3-5 Peel Boundary 断層ゾーンの水頭分布、涵養量と湧水量分布（Bense と Kooi,2004) .............................................................................................. 60

Figure 3-6 砂と粘土の互層を切る断層沿いの Shale Gouge Ratio (Bense and van Balen, 2004) ................................................................................................. 62

Figure 3-7 Plœmeur 帯水層を切る正断層と高透水ゾーン(Borgne et al, 2006) ...... 63

Figure 3-8 Nguyen ら(2007)が電気トモグラフィーを行った南フランスの断層 . 64

Figure 3-9 Micarelli ら(2006) の提唱する正断層の発達過程と水理性状 .............. 65

Figure 3-10 Aigio 断層の断面図と断層を貫くボアホール沿いの温度分布(Doan ら、2007) ............................................................................................................ 66

Figure 3-11 ゆるい傾斜の逆断層の可変揚水量試験 (Antonio と Pacheco, 2002) ... 67

Figure 3-12 高精度の反射波を使った地震探査によって特定された断層群(Carvalho et al., 2006) ................................................................................. 68

Figure 3-13 断層の垂直方向の高透水性による地下水のフッ素含有量の相違(Kim and Young Jeong, 2005) .............................................................................. 69

Figure 3-14 Doan ら(2006)による Chelungpu 断層の水の拡散率の測定の際のボア

ホール配置 ................................................................................................. 70

Figure 3-15 Blue Ridge Province の逆断層の高い貯留能力 (Seaton and Burbey, 2004)… ........................................................................................................ 71

Figure 3-16 モデル化した 3 種類の断層を横切る流れと物質移行（Bense and Person, 2006） ............................................................................................ 73

Figure 3-17 断層沿いの湧き水の温度分布と地球統計学手法を使ったシミュレー

ション(Heffner and Fairley, 2006) ............................................................. 74

Figure 3-18 断層のステップオーバー内の地下水の 3 次元的流れ(Fairley and Hinds2004a, 2004b) ..................................................................................... 75

Figure 3-19 Evans ら(1997)の採取した逆断層ゾーン内のコアサンプルの位置と方

向 ................................................................................................................. 76

Figure 3-20 シャーリー逆断層周辺の地下水の流れ(Johnson and Huntoon, 1994) .. 77

Figure 3-21 リオグランデリフトの断層ブロック沿いの地下水の流れ (Mailloux ら, 1999) ............................................................................................................ 78

Figure 3-22 不透水バリアとなっている正断層の両側の水頭と水温 (Hamaker, 2005) ............................................................................................................ 80

Figure 3-23 ユタ州 Timpoweap Canyon のハリケーン断層の断面図(Dutson, 2005) 81

Figure 3-24 断層の厚さと断層のバルクの透水係数の関係(Shipton ら、2002) ..... 82

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Figure 3-25 ユタ州エルクホーン断層の電気抵抗トモグラフィー解析結果 (Marlerと Ge, 2007) ................................................................................................ 83

Figure 3-26 テキサス州 Medina 群の断層にコントロールされた 4 つの地下水流(Clark and Journey, 2006) ........................................................................... 84

Figure 3-27 テキサス州、Hidden Vally 断層の露頭トレース（SWRI, 2008） ....... 85

Figure 3-28 ヤッカマウンテンの断層群(Bredehoeft, 1997) ....................................... 86

Figure 3-29 ヤッカマウンテン南西の断層による磁気異常のリニアメント(Blakelyら、2000) .................................................................................................... 87

Figure 3-30 Flint ら(2001)のヤッカマウンテンの不飽和部の概念モデル。断層が

早い水みちとなっている。 ..................................................................... 88

Figure 3-31 ヤッカマウンテン周辺の主要断層とトレンチの位置 (IAEA, 2002) .. 89

Figure 3-32 ネバダ州 Keno 断層ゾーンとスキャンライン(Hammond and Evans, 2002) ............................................................................................................ 92

Figure 3-33 詳細な断層ゾーンのマッピングと実データにキャリブレートされた

イメージデータに基づく透水係数を使った等価な数値モデル(Jourde et al., 2002) .................................................................................................. 93

Figure 3-34 NNW 断層を挟んだ両側の地下水頭応答挙動の違い(Takeuchi et al. 2007) ............................................................................................................ 94

Figure 3-35 Doughty and Karasaki (2003) がモデル化した 9km×9km×2km の東濃地

区のモデルの断面。月吉断層はサンドイッチ構造となっている。 . 95

Figure 3-36 低透水ゾーン断層モデルとサンドイッチ構造断層モデルの逆解析水

頭値分布の比較 (伊藤ら、2005) .............................................................. 96

Figure 3-37 石井ら (2006) が反射法地震探査と AMT 法を使って推定した大曲断

層の 3 次元構造 ......................................................................................... 97

Figure 3-38 LIDAR によるマップ（左）通常の空中写真（中）フィルターを掛け

て植生を取り除いた DEM、（右）断層トレースを重ねた図。http://geomaps.wr.usgs.gov/sfgeo/quaternary/stories/find_faults.html ....... 98

Figure 3-39 揚水試験に対する(a)低透水性 (b)高透水性 (c) 二面性の断層の反対側

の水頭応答分布(Anderson, 2006) ............................................................. 99

Figure 3-40 不均質な断層内の流れのパターン。(a)は母岩の平均透水係数 10-19m2、

(b)は 10-16m2のケース。(Lopez and Smith, 1996) ................................. 100

Figure 3-41 断層を跨いだ水理試験の数値モデル。4km×4km、10m 格子グリッド。

拡大図は断層の構造を示す。揚水井（矢印）、観測井（虫眼鏡）は

それぞれ断層から 100m の距離。 ........................................................ 101

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Figure 3-42 それぞれの断層のケースの揚水に対する圧力分布。断層がある場合

は何れの場合も断層の反対側に圧力が伝わりにくいことがわかる……………………………………………………………………………101

Figure 3-43 各ケースの圧力降下の時間変化。観測井では何れのケースも断層が

ないケースに比べて圧力降下度が低いことがわかる。 ................... 102

Figure 4-1 Schematic of conceptualizing fractured formation using the double-porosity conceptual model (Warren and Root, 1963). ............................................. 112

Figure 4-2 Schematic of conceptualizing vuggy fractured formation as a discrete fracture system with well connected, (a) outcrop pictures and (b) conceptual model (Wu et al. 2006). ............................................................................. 113

Figure 4-3 Space discretization and flow-term evaluation in the integral finite difference method (Pruess et al., 1999) ..................................................... 120

Figure 4-4 HDB boreholes at the Horonobe URL site (Yabuuchi et al., 2006) .......... 131

Figure 4-5 Comparison of simulated and measured temperatures at Well HDB-7 with water injected at bottom boundary at a rate of 0, 1 mm/yr, 10 mm/yr, and 50 mm/yr. ........................................................................................................ 136

Figure 4-6 Comparison of simulated and measured temperatures at Well HDB-7 with water injected at top boundary at a rate of 0, 3 mm/yr, 10 mm/yr, and 50 mm/yr. ........................................................................................................ 137

Figure 4-7 Comparison of simulated and measured pressures at Well HDB-7 with water injected at top boundary at a rate of 0, 3 mm/yr, and 10 mm/yr. ..... 137

Figure 4-8 Comparison of simulated and measured temperatures at Well HDB-8 with water injected at bottom boundary at a rate of 0, 6 mm/yr, 10 mm/yr, and 50 mm/yr. ........................................................................................................ 138

Figure 4-9 Comparison of simulated and measured temperatures at Well HDB-8 with water injected at top boundary at a rate of 0, 10 mm/yr, and 50 mm/yr. ... 139

Figure 4-10 Comparison of simulated and measured pressures at Well HDB-8 with water injected at bottom boundary at a rate of 0, 6 mm/yr, 10 mm/yr, and 50 mm/yr. ........................................................................................................ 139

Figure 5-1 断層の調査ステップの流れ ................................................................... 139

1

1 Introduction

The Nuclear Waste Management Organization of Japan (NUMO) and the

Department of Energy of the United States of America (DOE) established a cooperative

agreement in the field of radioactive waste management on July 10, 2002. In May 2005,

NUMO and the Regents of the University of California as the DOE Management and

Operating Contractor for the Ernest Orlando Lawrence Berkeley National Laboratory

(LBNL) entered into an agreement to collaborate, and for LBNL to conduct work under

the auspices of the bilateral agreement.

In 2006, ANRE (Agency for Natural Resources and Energy) jointly with JAEA

(Japan Atomic Energy Agency) identified outstanding technological issues and needs

regarding the research and development for geologic disposal of HLW subsequent to the

publication of the Second Progress Report by JNC (Japan Nuclear Cycle Development

Institute) in 1999. Research organizations in Japan as well as NUMO have been

conducting investigations on these issues and needs. In the area of groundwater

hydrology, four R&D needs were identified and are currently being investigated:

improvement of groundwater flow characterization technology, development of testing

and characterization technology in coastal areas, development of testing equipment and

technology, and field application of testing and characterization technology. NUMO has

been incorporating the results of the outcome of these R&Ds as they become available

and are deemed appropriate, and is in the process of systematizing the testing and

characterization technology to form a solid technical foundation for selecting the sites for

detailed investigation.

The first NUMO-LBNL collaborative project was entitled “Feature Detection,

Characterization and Confirmation Methodology,” which was designed to further develop

radioactive waste management technologies related to an investigation strategy and

technology for detection, characterization, and confirmation of key geologic features at

possible nuclear waste repository sites. The project was carried out from May 2005

through March 2007. Among other important findings, the study has identified the

hydrologic properties of fault zones as one of the most important parameters that need to

be evaluated during the preliminary investigation stage. Based on the lessons learned at

2

the Mizunami and Horonobe URLs, as well as at numerous mines, dams, and tunnels—

and given the geologic environment of the Japanese Islands—faults are likely to exist

almost ubiquitously, which need to be assessed both at the preliminary and the detailed

investigation stage (the length scale of the faults of interest would range from several

kilometers in the former down to several hundred meters in the latter). However, none of

the four R&D activities mentioned above sufficiently addresses the development of

systematized hydrologic characterization technology specifically tailored for fault zones.

At present, it is necessary to use perhaps overly conservative values for the hydrologic

parameters of fault zones for the design and performance assessment of a repository.

Therefore, development of a more efficient and reliable fault-zone characterization

technology is highly desirable. The geologic properties of faults and the relationships

among their geometry, type, fault parameters, and internal structures are being

investigated mostly overseas. Hydrologic investigation of faults of various sizes are also

being conducted at foreign as well as at domestic characterization sites. However, the

relationship between the geologic and hydrologic properties of faults is not yet studied

sufficiently.

The objectives of the present study is to organize the information available from

overseas to ultimately establish an efficient and systematized methodology for hydrologic

investigation and characterization of faults at the scale of interest during the preliminary

investigation stage, for more practical design and performance assessment. The present

study will be conducted as a collaborative study between NUMO and USDOE/LBNL. It

should be possible to apply/transfer the results of the study obtained at a site in the West

Coast of the United States, whose tectonic environment is just as active as that in Japan,

to the Japanese repository program when they will be needed. These actions would also

be beneficial to the U.S. program.

In addition to this introduction, the report is comprised of four main chapters: In

Chapter 2, we search and examine the literature that studies faults and their hydrologic

properties. We investigate the relationship between geologic attributes and hydraulic

properties of faults. We group and tabulate known geologic and hydraulic features of a

given fault by fault types. Chapter 3 surveys literature on characterization technologies of

fault zones and lists by the location including a couple of sites from Japan. In addition,

3

potential difficulties of correctly characterizing a fault by analyzing pressure transient

data during pump tests conducted in a formation containing a fault are discussed. The

chapter is intentionally written in Japanese to help NUMO personnel understand the

contents readily and clearly. In Chapter 4, we discuss conceptualization and modeling

approach of fault zone hydrology. We propose a numerical model that can represent

various coupled physical processes that are expected to take place in the fault zone. We

conclude with Chapter 5 that discusses the characterization strategy of fault zones. We

make a step-by-step recommendation for effective characterization of fault zone

hydrology during the preliminary investigation stage.

4

2 Geological Structure of Fault Zones and Hydrology

2.1 Introduction

Fault zone hydrology, a key issue for many different geological disciplines, has in

recent years been the subject of increasing interdisciplinary effort. For example, faults

have been studied as part of research regarding (1) ore deposits associated with

geothermal migration (Hammond and Evans, 2003); (2) groundwater management

(Moran and Hudson, 2000; Stamos et al., 2003; Nishikawa et al., 2004); (3) petroleum

migration (Antonellini and Aydin, 1994, Dholakia et al., 1998; Sorkhabi et al, 2002a,b,

Takahashi 2003a,b, Jourde et al, 2002); (4) geotechnical projects, such as tunneling and

underground construction (Yanagisawa et al., 1995); (5) fault slip propagation (Sibson,

1981); and (6) nuclear waste disposal (Onishi and Shimizu, 2002; JNC, 2000; Davison

and Kozak, 1988; Le Cain et al., 1998; Swedish Nuclear Fuel, 2005; Henriksen, 2008,

Andersson et al., 1991).

Structurally, over regional or local tectonic history, faults and fractures are

responsible for rock-mass weakness. Faults and fractures are also the main pathways for

fluid, gas, and heat, greatly enhancing hydrogeological properties. With respect to fluid

flow, a fault with distinct hydraulic properties may act as a conduit, a barrier, or both

(Caine et al, 1996).

Permeability structures of fault zone have been reported in the literature, largely

through investigations of natural faults (Evans et al., 1997; Caine and Forster, 1999;

Caine et al., 1996; Onishi et al., 2002; Wibberly and Shimamoto, 2003), using outcrop

and small core samples for permeability measurements. The main limitation to this

method is that large faults and fractures are not taken into account, and therefore

permeability values can be underestimated. Allowing for this limitation, these studies

suggest that the permeability structure is related to fault evolution (i.e., geologic time,

reactivations), stress field, juxtaposition of rock types, amount of displacement, depth

(temperature and pressure), fluid-rock interaction, fracture interconnectivity, fault-rock

heterogeneity, and the extent of fracture filling by mineral precipitation. These factors

affect fault distribution, as well as the length and width of the fault zone.

5

This report summarizes the fault hydrology studies now available in the literature.

We point out the importance of fault zone hydrology, discuss its complexity, and make

recommendations for future work, including analogue studies in the USA and

characterization strategies. In more detail, we present several tables that show each type

of fault (i.e., normal, strike-slip, reverse/thrust) with structural and hydrologic

descriptions for reference.

2.2 Fault Anatomy

Faults are generally defined as a planar or zonal structure, which accommodates

shear displacement. Faults are sometimes considered as planes or surfaces (Hobbs et al.,

1976), slip planes, or shear fractures (Aydin, 2000). Indeed, most faults are defined by

two primary components: damage zone and fault core (Caine et al., 1996; Chester and

Logan, 1986). In general, the deformation process operating in the damage zone produces

a wide range of fracture networks, the presence of small faults, folds, veins, and

deformation bands. Fracture density in the damage zone can be three to five times greater

than in the host rock (Evans et al., 1997).

The fault core, where the deformation is concentrated, can be very heterogeneous.

It consists of breccia, cataclasite, or gouge, in crystalline rock; and sheared layers of

weak lithologies (such as clay smearing, grain scale mixing, grain reorientation and pore

collapse, cataclasis with deformation bands) in lithified and poorly lithified sedimentary

rock. The width of fault core can vary from a few millimeters to a few centimeters.

These principal fault zone components are encompassed by the protolith, or

undeformed host rock. Boundaries between the damage zone and fault core are typically

sharp, whereas the damage zone to protolith transition is usually gradational (Caine et al.,

1996). The amount and distribution of each component depend upon the lithology and the

geologic setting in which the fault has formed (Caine et al., 1996, Evan et al., 1997).

Caine et al. (1996) classified fault zone architecture and permeability structure

according to the percentage of damage zone versus the percentage of fault core. This is a

simplified conceptual model that includes structural and hydrological characteristics. As

a result, Caine et al. have identified four end members: localized conduit, distributed

6

conduit, combined conduit-barrier and localized barrier. Their conceptual model shows a

qualitative relationship between fault internal structure and fault permeability, based on

outcrop observation and permeability data from laboratory measurements. However,

faults are structurally and hydrologically heterogeneous. Therefore, their model does not

address current fault-rock interaction, flow paths, or flow properties. As indicated by

Caine et al. (1996), the simplified conceptual model represents only one stage in the

temporal evolution of a fault. Larger faults, such as the Median Tectonic Line and

Carbonaras Fault, are more complex and do not fall into this classification (Faulker et al.,

2003; Wibberley and Shimamoto, 2003).

It is well known that the permeabilities of rock obtained from outcrop are two

orders of magnitude higher than those of rock obtained from drill cores (Morrow and

Lockner, 1994). The permeability of exhumed fault rock tends to display less pressure

sensitivity owing to the presence of weathering products in fractures. Nonetheless, core

plug scale tests aid in understanding the permeability structure of fault zone components,

although permeability values should be used with caution in developing quantitative

models of fluid flow. In situ permeability tests would probably yield higher permeability

values, given the interconnectivity of fractures that are missed in core plug scale

investigations. It is evident that insufficient downhole data from across the fault zone

exists in the literature, as described in the following sections.

2.3 Structural characteristics of faults in various rock types

2.3.1 SEDIMENTS AND SEDIMENTARY ROCKS

In connection with oil exploration, investigators have studied the impact of faults

in sedimentary rock over a wide range and degree of lithification in sedimentary basins

(Antonellini and Aydin, 1994).

One of the main characteristics of faults in sedimentary rock is the formation of

deformation bands, i.e., the fault damage zone consists of deformation bands. Idealized

representations of fault architecture in sandstones often include a fault core composed of

low-permeability fault rock, high-permeability slip surfaces, and a surrounding damage

zone (Figure 2-1). In such scenarios, the fault rock extent and permeability are crucial for

7

determining the fault’s perpendicular flow component. Slip surfaces, the most continuous

structural element, often control fault-parallel flow and are commonly treated as two

planar surfaces separated by a constant overall aperture.

Figure 2-1. Deformation bands at Estrada Sandstone. Fossen and Bale, 2007

According to Fossen and Bale, 2007, deformation bands form in sediments with

porosity higher than 16%. In sandstones, they occur as single structures, as clusters, and

in fault damage zones. Deformation bands are millimeter-thick tabular zones of localized

strain. They deviate from regular fractures by their lack of a single, continuous slip

surface. For this reason, they do not share all of the properties of regular fractures

(Fossen and Bale, 2007). In sediments rich in phyllosilicates, such as mica and clay,

deformation bands will be formed by alignment of those minerals to form fabric parallel

to the band (Knipe et al, 1997). Aydin and Johnson (1978) reported a different type of

deformation band, a cataclastic band. In such bands, grains are crushed (cataclasis) and

the grain size is consequently reduced.

Cementation and dissolution at grain-grain contact points can reduce porosity in

any type of deformation band. Deformation bands may be preferentially cemented

8

because of their fresh grain surfaces formed during frictional grain sliding and

comminution. Many factors influence the deformation mechanism and, hence, the type of

deformation band that is formed, including mineralogy, grain size, grain shape, grain

sorting, degree and type of cementation, porosity, and confining pressure (Fossen and

Bale, 2007).

Shear smear is another term commonly associated with a fault core rich in clay

minerals. It is observed when the fault displaces layers of sandstone and shale.

Laboratory experiments by Takahashi (2003a), using interlayered sandstone and shale,

suggested that during normal fault deformation, faults are sealed by shale smear

impacting the mechanics and hydrologic properties of the fault zone. Field observations

by Hynekamp et al. (1999) and Eichhub et al. (2005) described a narrow core zone

flanked by a wider footwall and hanging-wall mixed material, suggesting that sediments

transported during normal fault-shearing are thinned and tectonically mixed within the

core zone. Transport distances are greater for clay and less for sand.

2.3.2 CRYSTALLINE AND VOLCANIC ROCKS

Crystalline rocks in this report refer to any type of rock that was originated by

magmatic or metamorphic processes and where minerals are visible to the naked eye. The

most commonly studied crystalline rocks are granite, granodiorite, gneiss, and schists. As

for volcanic rocks, welded and nonwelded tuffs have been extensively studied within the

Yucca Mountain Program (USA). Faults in crystalline rocks have been studied in

connection with mineral and geothermal exploration, tunneling, and nuclear waste

disposal programs (in such countries as Canada, Sweden, Finland, and Japan).

The observation of ore deposits filling veins in the proximity of fault zones

indicates paleo-fluid migration during fault activity. Eichhubl and Boles (2000, 2001)

evaluated mineral precipitation in veins to infer fluid volume and chemistry. Because the

fractures and faults were sealed at the times of these studies, fractures and faults are in

this case most likely to act as barriers to fluid flow.

All types of faults occur in crystalline rock, one example being Dixie Valley,

Nevada, an active normal fault studied for geothermal exploration. Thrust faults with

9

large displacement are often observed in stable cratons in orogenic belts, such as at Lac

Du Bonnet batolith in Canada, the Finnsjon and Forskmark sites in Sweden, and the East

Fork, Wyoming and Elkhorn, Colorado faults in the USA. Such sites usually show a

combination of ductile and brittle deformation, caused by formation depth, followed by

subsequent uplift and fault reactivations in brittle regimes. At sites in Canada and Sweden,

faults zones are defined as deformation zones, because faults are more complex and the

deformation diffuse. The Nojima and Tsukiyoshi faults in Japan are examples of active

and inactive reverse faults, respectively.

The majority of the hydrologically characterized strike-slip faults around the

world are from regions located along plate boundaries. These include several strands of

the San Andreas Fault, which juxtapose granodiorite with metarmorphic and sedimentary

rocks such as at Punchbowl, San Gabriel, Gemini, and Bear Creek Camp faults.

Investigations of the Median Tectonic Line in Japan and the Carbonara Fault in Spain are

found in metamorphic rocks.

One characteristic of faults in crystalline rocks is the presence of variable amounts

of breccia, gouge, and cataclasite (Figure 2-2). These terms are used for rocks formed

from shearing under brittle conditions, at depths less than 10–15 km. Faulted rocks

greater than 15 km depth occur under ductile conditions, where milonites are formed.

Another well-known characteristic of faults cutting across crystalline rock is the

presence of micas and clay minerals in the fault core, resulting from large amounts of

silicate minerals reacting with fluids and comminuted minerals as a result of shearing.

Hence, a significant reduction in permeability is observed in studies conducted in such

lithologies (Evans et al., 1997; Forster et al., 1994; Seront et al., 1998; Forster and Evans,

1991; Onishi et al., 2002; Wibberley and Shimamoto, 2003; Lockner et al., 2000;

Mizoguchi and Shimamoto, 2001; Forster et al., 2003).

10

Figure 2-2 (a)- Strike-Slip Punchbowl Fault (inactive strand of the San Andreas Fault) juxtaposing sandstone and granite; (b) photo showing ultracataclasite (from field guide to Punchbowl Fault, Chester, 1999)

2.4 Survey of past and current hydrologic characterization activities of fault zones

According to the classification system devised by Anderson (i.e., “Andersonian

classification”), there are three fundamental fault types: normal, thrust or reverse, and

strike-slip. Each of these fault types can combine to form a more complex fault system.

In theory, considering the tectonic regime that generates these types of faults, we

would expect the hydrologic regime to be controlled by the orientation of open fractures

formed by tensional forces acting on the rock mass (Neubauer, 2005). In such a case,

a

Punchbowl Fault

b

ultracataclasite

11

reverse faults would be straightforward with respect to fluid flowing upwards, because

most tensional fissures are subhorizontal and often not interconnected. By contrast,

strike-slip faults have subvertically oriented fissures that would enhance fluid flow

circulation. However, such models are not realistic, as we find when we compare several

examples of fault hydraulic properties collected from the literature. This might be caused

by spatial heterogeneity in the fault system, the location and depth of formation, the

mechanical properties of different lithologies, variations in the stress field with depth, etc.

Below, we describe the main characteristics gathered from our literature survey for each

type of fault.

2.4.1 NORMAL FAULTS

Normal faults are associated with crustal extensions and mostly with basin

formations. As described in Section 2.3.1, they are important structures for hydrocarbon

reservoirs. Normal faults zones in sedimentary rock have been described in a number of

recent studies—Devatzes et al. (2005); Antonellini and Aydin (1994, 1995); Antonellini

et al. (1994); Eichhubl et al. (2005); Fossen and Bale (2007).

The hydraulic properties of deformation bands have been widely described, using

numerous laboratory measurements and well log studies. Porosity loss and pore-throat

collapse in deformation bands reduce permeability (Antonellini and Aydin, 1994;

Antonellini et al., 1994; Fisher and Knipe, 1998; Matthäi et al., 1998). Permeability

within deformation bands is about three orders of magnitude lower than in the host rock

(Pittman, 1981; Antonellini and Aydin, 1994, 1995; Shipton et al., 2002; Taylor and

Pollard, 2000), and porosity is less than 1% (Shipton and Cowie, 2003). Usually the

12

hydraulic conductivity of a fault core is reduced as a result of clay smearing, grain-scale

mixing, and iron-oxide precipitation (Bense et al., 2003). Antonellini and Aydin (1994)

reported that cataclastic deformation may reduce sandstone permeability by as much as

104 (to 10-16 to 10-19 m2) within fine-grained deformation bands (Figure 2-3).

However, in eolian sandstone, such as described by Antonellin and Aydin (1994)

and Shipton et al. (2002), the bulk permeability is higher than in the deformation bands

by up to three orders of magnitude. These are observed in several examples shown in

Table 2-2 including the Sand Hill Fault, the fault at Arches National Park, the Big Hole

fault, and the fault at Vale of Fire State Park. In addition, joint openings or slips across

established joints, which produces dilation caused by surface roughness along fracture

walls (Brown, 1987), increases the permeability of joints in sandstone (Taylor et al.,

1999).

In the Hazel-Atlas fault, Eichhub et al. (2005) found that deformation during the

evolution of the fault involved a complex mixture of clay mineralogy reducing its

permeability. The change in clay fabric—by distinct increase and aligning parallel to the

fault plane—suggests that the fault acts as a barrier to fluid flow across itself.

Bhrun et al. (1994) compared three normal faults in similar granitic rock at

various sites: Dixie Valley, Nevada; Wasatch, Utah; and Mineral Mountain, Utah. These

sites were all intensely fractured and hydrothermally altered, and they also all had

transition zones (i.e., damage zone) in the footwall composed of variably fractured and

altered rock. The transition zones were spatially heterogeneous, with thicknesses ranging

from 20 to 200 m. The slip zones ranged from <10 m in the Mineral Mountains to 20 m

in parts of the Dixie Valley and Wasatch fault zones. Although the normal fault zones

displayed similar gross zonations, characteristics of fracture networks varied with depth.

Permeability measurements by Seront et al. (1998) found permeability anisotropy

from outcrop samples. Their study (in granodirite) suggested lower permeability in the

core (cataclasite, implosion breccia, and breccia). They noted that permeability decreased

with increasing confining pressure over a range of four orders of magnitude (from 10-16

m2 to 10-20m2). However, because of the high degree of alteration, great hysteresis during

pressure cycles was observed when compared with relatively unaltered granites.

13

In another study, Caine and Forster (1999) simulated fault zone permeability in

3D based on fracture data from a series of normal faults in low-permeability rock.

Because strike-slip and thrust faults contain similar architectural elements, the model

results from Caine and Forster’s study may be applicable for those faults. Their findings

suggest fault permeability anisotropy is controlled by a fracture network’s internal bulk

anisotropy (e.g., related to aperture, fracture roughness, and anisotropy in the fracture

topography), and the lithological contrast when juxtaposing different fault-zone

components. The permeability anisotropy in three orthogonal flow directions suggested a

lower permeability across the fault of up to five orders of magnitude when compared to

the two directions parallel to the fault plane. Similar results were found by bulk

permeability of Stillwater fault zone. The bulk permeability of the fault zone probably

exceeded that of the protolith by at least five orders of magnitude, and likely exceeds the

permeability of breccia pods in the fault core by at least seven orders of magnitude

(Seront et al., 1998).

14

Figure 2-3

Figure 2-4 shows a schematic representation of deformation-band hydraulic

characterization. As shown in Figure 2-4 deformation-band faults can be an important

barrier to fluid flow in hydrocarbon reservoir rocks. Conversely, it has been suggested

that an increase in permeability is created in the direction parallel to slip because of

dilatancy and slip-surface generation (Antonellini and Aydin, 1994).

15

Figure 2-4 Deformation bands at Estrada Sandstone. Fossen and Bale, 2007

The Yucca Mountain Project, the investigation into a proposed nuclear-waste

repository in Nevada, is one of the best examples of studies in volcanic rock, specifically

hydraulic studies in welded and nonwelded tuffs. Yucca Mountain is dominated by a

series of north-striking normal faults, with bedrock displaced several hundred meters

(maximum ~600 m) along many of the faults, which occur within or along the flanks of

Yucca Mountain (Fenster, 1999). Hydrologic data on fault zones in this area was limited.

Air injection tests and trace tests were conducted along the Ghost Dance and Bow Ridge

faults at this site to determine air permeability, porosity, and tracer transport (BSC, 2004b,

Section 2.2.3). According to LeCain et al. (1998), porosity and permeability in the fault

core was up to one order of magnitude higher than in the host rock. Faults could either be

major conduits for flow or may be locally impermeable to lateral flow, resulting in

perched water (Flint et al., 2001).

Fault-zone studies of ore deposits suggest some evidence that fluid migration is

confined to one side of the fault (Logan and Decker, 1993; Hammond and Evans, 2003,

Diment and Craig, 1999). Examples include mineralization exclusively in the hanging

wall in Coastal Somerset, UK; gold mineralization in South Keno Fault, Nevada, and

Tritina Fault, Yukon; and gold mineralization in the footwall North Keno Fault. This

selective mineralization is possibly due to structural control and the nature of repeated

deformation events that enhance fault dilantacy.

17

Table 2-2 N

ormal Faults

18

2.4.2 THRUST AND REVERSE FAULTS

Thrusts faults are gently (~30 o) dipping faults in which the hanging wall has

moved up relative to the footwall (Hatcher, Jr., 1995), while reverse faults are those with

moderate-to-steep dip (more than 45o). Both result from tectonic compression or

shortening.

Thrust-fault hydraulic properties from field, laboratory, and modeling studies

provide insights into permeability anisotropy within fault-zone components. Anisotropy

in permeability was observed in a number of studies (Forster and Evans, 1991; Evans et

al., 1997; Locker et al., 2000; and Onishi et al., 2002), in which the low-permeability

fault was localized in the clay-rich gouge/cataclasite, while higher permeability was

found in faulted and fractured rocks in the damage zone. Permeability in the damage zone

was up to six orders of magnitude higher than in the fault core (Evans et al., 1997), as

shown in Figure 2-5. In outcrop samples tested in three perpendicular orientations by

Evans et al. (1997), permeability anisotropy was clearly observed. In the studies covered

by our report, which were conducted in exhumed fault zones in crystalline rock, it was

noted that a gouge and/or cataclasite component act as a barrier to flow across the fault,

whereas the damage zone enhanced flow within the plane of the fault. Similar results

were obtained using core samples from the Tsukiyoshi fault in Japan. Onishi et al. (2002)

report permeability as low as 10-19m2 in the foliated cataclasite, with anisotropy in the

damage zone ranging from 10-15 to 10-19m2.

Table 2-2 N

ormal Faults (continued)

19

Figure 2-5 Shows the distribution of permeability across fault zone from Evans, Forster, Goddard, 1997

Two examples of permeabilities from slug and pumping tests in thrust and reverse

faults, at the Elkhorn and Tsukiyoshi faults, suggest higher permeability values compared

to laboratory measurements. In both cases, the permeabilities of the protolith and damage

zone were found to be two to five orders of magnitude higher than laboratory values. See

Table 2-4.

21

Table 2-4 Thrust/reverse faults

22

Table 2-4 Thrust/reverse faults (C

ont.)

23

2.4.3 STRIKE-SLIP FAULTS

Strike slip faults are faults those in which the motion is parallel to the strike of the

fault plane. They are characterized by steep dip and predominantly horizontal

displacement. Most dips are more than 60º, and many approach vertical. In terms of

tectonic regime, strike-slip faults have sub vertically oriented tensional fissures

subparallel to the strike of the fault, as described previously (Neubauer, 2005). This

orientation would allow the circulation of descending and ascending fluids parallel to the

strike and dip of the fault plane.

In eolian sandstone, deformation bands reduced permeability by two to four

orders of magnitude according to gas permeameter measurements (Freeman 1990) and

two to four orders of magnitude lower than host rock according to gas injection

minipermeameter measurements (Antonellini and Aydin, 1994). In addition, a 1.3-to-2.3

orders-of-magnitude reduction was observed in deformation bands (when compared to

host rock) from field measurement of relic fluid gradients (Taylor and Pollard, 2000).

Most examples of strike-slip faults occur at plate boundaries. Examples include

the San Andreas Fault and its strands in the USA; the Median Tectonic Line and Nojima

Fault in Japan; the Carbonaras Fault in Spain; and the West Fissure Zone in Chile. The

San Andreas Fault is one of the most well-known fault systems in the world. The fault is

composed of numerous strands that run parallel to the main fault, some active and others

inactive. Petrological, structural, and geophysical studies have been conducted at several

areas along the San Andreas Fault; however, there remains a lack of hydrological

information on fluid flow across the fault.

24

Faults in unconsolidated sediments are summarized in Table 2-3. Most hydraulic

studies in unconsolidated sediments were found to be in alluviums, as part of

groundwater management efforts for drinking water programs. For the purpose of water

management, a number of wells have been drilled at shallow depth. Moran and Hudson

(2000), Stamos et al. (2003), and Nishikawa et al. (2004) describe faults cutting through

aquifers. Their hydrology was characterized by geochemical and isotopic signatures. In

all cases, faults in unconsolidated sediment were found to reduce the hydraulic

conductivity perpendicular to the fault.

Several strands of the San Andreas Fault showed changes in fault movement and

dip direction due to reactivation and/or change on stress regime. Such changes in fault

movement were observed in Japan by Onishi, (unpublished data from the Tsukiyoshi

fault), from reverse to strike-slip, by analyzing slickensides on fault core; and by Hidemi

et al. (2000) at the Nojima fault, a strike-slip fault that was recently reactivated as a

reverse fault during the Kobe earthquake.

25

Table 2-5 Strike-slip faults

26

Table 2-5 Strike-slip faults(C

ont.)

27

T

able 2-5 Strike-slip faults(Cont.)

28

T

able 2-5 Strike-slip faults(Cont.)

Table 2-5 Strike-slip faults(C

ont.)

29

2.5 Scaling relationship between fault components

Over the past two decades, studies in fault zones have also included the estimation

of fault components and scaling relations. By using the distribution of faults, it was

possible to estimate a linear interrelationship between (a) displacement and length, (b)

fault width and length, and (c) fault width and displacement.

In the literature, it is observed that the amount of displacement (d) on a fault from

natural faults is proportional to the mapped trace length (L), based on the fault scaling

relationship d=cLn , where 2 ≥ n ≥1 and c is a variable that depends on rock properties

and n a controversial exponent, related to the least-squares best-fit linear relationship

(Cowie and Scholz, 1992).

Data relevant to the scaling relationship has been evaluated in all types of faults

(i.e., normal, thrust, strike-slip), in a variety of tectonic environments, and for a range of

length scales (Cowie and Scholz, 1992a and reference therein; Schultz and Fossen, 2002;

Janssen et al., 2002). As a result of data plotting, Figure 2-6 and Figure 2-7 show log-log

plots (derived from data plotting) of maximum displacement and length data for a wide

range of tectonic environments. For example, faults tens of kilometers long typically have

displacements on the order of hundred of meters, while faults of a few meters in length

have offset on the order of only a few centimeters.

30

Figure 2-6 Compilation of displacement-length data in 2D. DB=deformation bands (Schultz and Fossen and reference therein, 2002).

Figure 2-7 Log-log displacement–length from Schilische et al. (1996).

Another scaling relationship is related to fault length and fault-zone width.

Janssen et al. (2002 and reference therein) predict that longer faults should have wider

fault widths. Based on the cohesion-zone fault growth model of Cowie and Scholz

(1992b), Scholz et al. (1993), postulated that the ratio between the width of the fault (Wf)

and the length of the fault (L) is approximately 0.25/0.2. Vermilye (1996) and Vermilye

and Scholz (1998) estimated a ratio of 10-2 for several meter scale faults. The ratio of

fault width (w) to fault length calculated by Janssen et al. (2002) falls within an order of

10-2 (Figure 2-8). Jenssen et al. (2002) used estimates of fault lengths and widths from

aerial photography, which fit quite well when compared with measured ratios from other

natural faults (Figure 2-8).

31

Figure 2-8 Plot of process zone (fault thickness) width versus fault length for this study and additional published data (Brock and Engelder, 1977, Cheraysbev and Dearman, 1991, Little, 1995 (Jenssen et al., 2002)

In addition, as shown in Figure 2-9, Otsuki (1978) (in JNC, 2000), within major

strike-slip faults all over the world, found a linear relationship between cumulative

displacement along the fault and the width of the shear zone. In this case, the shear zone

of a fault with a cumulative displacement of tens of kilometers is estimated to be 200 m

in width.

32

Figure 2-9 The relationship between width of shear zone and displacement for strike-slip faults

Although there is still controversy over whether all faults show a linear

relationship between displacement, length, and fault width, it is generally agreed that

such a linear relationship holds for larger faults, i.e. lengths of tens of kilometers. Schultz

and Fossen (2002) noted that the scaling relationship does not fit quite so well for

deformation bands on a meter scale, as observed in Figure 2.6.

2.6 Features to consider for hydraulic characterization of fault zones:

As reported in the literature, faults are complex structures. To effectively

characterize fault-zone hydrology, we must take many factors into consideration. One

important observation is that faults need to be evaluated in three dimensions, and possibly

four. In this case, time (the fourth dimension) is an important variable that can affect the

structural and hydrological behavior of faults. Below, we list the key factors when

characterizing faults:

- Lithological variation (stratigraphy for sedimentary rocks and compositional

variations for crystalline rocks)

1. San Andreas fault 2. Alpine fault 3. Great Glen fault 4. Garlock fault 5. Bok Bak fault 6. Death Valley-Furnace Creek fault 7. Hizune-Hasenuma fault 8. Akaishi Tectonic line 9. Hope fault 10. Agua Blanca fault 11. Atera fault 12. Blue Cut fault 13. Nukumi fault 14. Tsurukawa shear zone 15. Oppara fault 16. Sekihara-Yanagase fault 17. Atotsugawa fault 18. Hirose-Hase fault 19. NS trending faults shift Kannawa fault 20. Tanna fault 21. NS trending faults shift Kannawa fault. 22. Toyano fault 23. Sahata fault 24. Hanaore fault

33

- Lithological composition (mineralogy, texture, morphologic features of cements,

percentage of clay minerals)

- Fault geographic distribution

- Fault internal geometry

- Fault structural evolution in space and time

- Fault history (age, formation depth, i.e. ductile or brittle regime and fault

reactivations)

- Fluid-fault interaction (mineral precipitation, alteration)

- Fluid chemistry

- Fault length and displacement

- Changes on stress field (past and present)

- Change in climate conditions ( precipitation, tectonic rebound due to ice melting)

- Deformation mechanism affecting the physical properties of rock, such as

porosity and permeability

- Fluid and pore pressure, aperture, fault weakening or hardening related to

episodic process of the seismic cycle

- Fracture network and connectivity in the damage zone

- Temperature and pressure, deformation rate

- Cyclic fault displacement and fluid migration: fluid migration and mineralization

possibly alternates with periods of slip and rupture

2.7 Analogue studies in the USA

The main purpose of conducting analogue studies in the USA is to perform field

studies at proposed site (s), to examine if the geologic properties and features of faults

can be used to predict the fault-zone hydrological properties.

2.7.1 SUGGESTED TARGET STUDY AREAS IN THE USA

In this section, we list three potential U.S. sites, two in California and one in

Colorado. The two sites in California are in Mesozoic to Cenozoic sedimentary rocks

featuring strike-slip and normal faults, whereas in Colorado, a thrust juxtaposes

sedimentary and granitic rocks.

34

2.7.1.1 CALIFORNIA STATE

The benefits of conducting analogue studies in California are as follows: (a)

geological similarities with Japanese geology, i.e., rock types; (b) similar tectonics (i.e.,

convergent plate margins and active plate margins for the past 230 Ma); and (c)

compressional and transpressional regimes.

Based on geology and tectonics, California is well suited for analogue studies on

fault hydrology. The advantage of conducting field testing in California is the easy access

to field location, data availability, cost, maintaining good public relations, and

strengthening the US-Japan collaboration.

Site A—Lawrence Berkeley National Laboratory The main characteristics that make this site appealing for analogue study are the

location inside the LBNL property, which is cut by several well-known strike slip faults.

The site also has the advantage of hundreds of drill-core data containing hydrological

information. Figure 2-10 shows the location of the site and the distribution of the main

fault zones.

Figure 2-10 Location of LBNL site (from Google maps)

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The bedrock at LBNL consists primarily of Cretaceous marine mudstone, shales,

and sandstones of the Great Valley Group, Miocene sedimentary nonmarine sandstone,

mudstone, and conglomerates of the Orinda Formation, and volcanic rocks of the Moraga

Formation. These units form a faulted homocline. The Cretaceous sequence forms the

structurally lower portion of the homocline, which is overlain by the Orinda Formation

and Moraga Formation, respectively (LBNL, 2000). Figure 2-11 illustrates the geology of

the LBNL site, and Figure 2-12 cross section of the site.

Figure 2-11 Distribution of geological formation at LBNL site

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Figure 2-12 Geological cross section shown in Figure 2-11

During a day field trip at the LBNL site (led by geologist Preston Jordan, who has

mapped in detail part of the lab site), several sites with faults were visited, including the

Wild Cat and East Canyon Faults. Although they are not exposed, documentation and

pictures of the fault zone are available from road and building construction and nearby

well data.

Wild Cat and East Canyon Faults are minor, secondary splay faults associated

with the Hayward Fault. The Hayward, a major branch of the San Andreas Fault system,

is a zone of highly deformed rocks that trend N30 degrees west from an area southeast of

San Jose to the San Pablo Bay. It exhibits creep along at least 68 km of its 100 km length,

and its width ranges from 2 to 10 km.

Wild Cat and East Canyon Faults cut late Cenozoic strata, striking subparallel to

the Hayward Fault. The Wild Cat Fault passes along the eastern margin of LBNL.

Regionally, the fault is difficult to map throughout its length and appears to be

discontinuous, although it clearly truncates and offsets strata at many locations. At LBNL,

it juxtaposes the San Pablo Group (fossiliferous marine sandstone) and Claremont

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Formation (marine siliceous rocks), and the strata adjacent to the fault have been severely

disrupted by steep, east-to-northeast dipping subsidiary faults. The fault is inactive in this

area, and studies suggest that it displaced the Moraga Formation by approximately 6 km

northwest.

The trace of the East Canyon fault is based on historic spring locations, air photo

analysis, and mapping. Apparent offset of the Orinda Formation (nonmarine sediments

such as mudstone sand to fine-medium grained sandstones and conglomerates) and

slickensided surfaces consistent with the orientation of the East Canyon Fault were

observed in trenches along the fault trace.

As illustrated in Figure 2-13, three main locations were selected for possible

analogue studies, based on geological and hydrological characteristics.

Figure 2-13 Proposed locations of trenches

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Site B – Black Diamond Mine The Black Diamond Mine is located 36 miles (23 km) east of Lawrence-Berkeley

National Laboratory and 45 miles (28 km) from downtown San Francisco (Figure 2-14). It

is part of the Regional Park of California State.

Figure 2-14 Location of Black Diamond Mine and distribution of main faults

Geologically, it is composed of Tertiary alternation of shallow marine sediments,

a mixture of shoreface sandstone, offshore marine shales, and distal muddy turbidite

deposits. The main formation observed at Black Diamond is the middle Eocene

Domengine formation. A day visit with NUMO in October 2007 included a stop at the

Hazel-Atlas normal fault described by Eichhubl et al. (2005). The site was primarily

mined for coal beds intercalated with well-sorted sandstone, which was also mined for

39

glass industry. The mine is no longer active, and most of the shafts are open for visiting

and scientific investigation.

The intercalation of white sandstone and shale, the presence of a fault, the support

of scientific investigations in the mine, the access and proximity to LNBL—all make it

one of the possible candidates for analogue study.

The Hazel-Atlas fault by Sullivan et al. (2003), shown in Figure 2-15, offsets a

sandstone-shale sequence that is part of the 230–260-m (754–853 ft) thick Domengine

Formation. The Hazel-Atlas fault displaces the sandstone-shale sequence by a dip

separation of 9 m of normal fault. Shale smear is observed along the fault plane, where it

deforms and attenuates to a few centimeters (Eichhubl et al., 2005).

Figure 2-15 Picture showing the Hazel-Atlas fault in the Black Diamond Mine, Antioch, CA

2.7.1.2 COLORADO STATE (POSSIBLE COLLABORATION)

Elkhorn Fault, South Park, Colorado (Figure 2-16) A research project led by University of Colorado is focused on the Elkhorn fault,

to investigate the hydrological properties of an exhumed thrust fault. Funded by the

40

National Science Foundation, this project assesses the impact of fault on regional

groundwater flow. The Elkhorn fault juxtaposes Pre-Cambrian granitic rock in the

hanging wall with sedimentary rocks in the footwall. Ongoing characterization studies in

the Elkhorn fault are being conducted by the University of Colorado and USGS scientists.

Preliminary estimations of fault-zone permeability were presented at the AGU Fall

meeting 2007 (Figure 2-17). In this study, geological, hydrological, geophysical, and

numeral modeling is being used to characterize the hydrology of the fault zone. A

possibility of collaboration is been suggested. The results from this paper presented at

AGU and previous studies are shown in Table 2-4.

Figure 2-16 Location of Elkhorn Fault, Colorado

41

Figure 2-17 Geology and location of wells across the Elkhorn Fault (by http://www.agu.org/meetings/fm07/fm07-sessions/fm07_H23A.html)

2.8 Summary

1) Table 2-2 Normal FaultsTable 2-4,Table 2-5 list the main results from our

literature survey regarding fault structural and hydraulic properties, separated by

type of fault. Table 2-2 Normal Faults lists the main characteristics of normal

faults, Table 2-2 the main characteristics of thrust and reverse faults, and Table

2-3 the main characteristics of strike-slip faults.

2) The overall conclusions of our fault study are that no matter what type of fault

(i.e., normal, thrust, or strike-slip) or lithology (i.e., lithified sedimentary,

crystalline, or welded volcanic rock), whether during inactive or interseismic

deformation; the fault core will act as a barrier to fluid flow normal to the fault,

while open fractures in the damage zone would act as a conduit (either parallel

to the strike and dip) of a fault. However, the fault core would act as a short-

lived, syn-deformational fluid-flow conduit during coseismic deformation, when

there is an increase in pore pressure and changes in the elastic properties of rock.

42

3) Except for elolian sandstone, which might have higher matrix porosity and

permeability than the deformation bands and fault core, most hydrological

studies in fault zones within low-permeability rock suggest a semi-impermeable

fault core, a highly permeable and conductive damage zone, and a low-porosity

and low-permeability protolith. The permeability in the damage zone is found to

be about two to three orders of magnitude higher than in the protolith, and the

fault-core permeability is up to seven orders of magnitude lower than that of the

damage zone. Such results implicitly suggest that faults are anisotropic.

4) Hydraulic properties in crystalline rock have been measured in laboratory, field,

and modeling studies. Using different methodologies, we found that a pattern

emerged when comparing permeability values from laboratory measurements. In

general, the permeability of a protolith or host rock ranges from 10-14 to 10-16 m2

at 4 to 30 MPa confining pressure. At much higher confining pressure, the

permeability reaches a minimum, i.e. 10-21 m2. In damage zones, the

permeability varies according to fracture density, ranging from 10-12 to 10-19 m2

over a wide range of confining pressures. In the fault core, the permeability

ranges from 10-16 to 10-20 m2, with confining pressures up to 200 MPa.

5) Hydraulic properties obtained from exhumed rock samples tend to display less

pressure sensitivity, due to the presence of weathering products. Furthermore,

tests on samples from any fault zone represent only one stage in the temporal

evolution of the fault.

6) The permeability of rock obtained from outcrop is up to two orders of magnitude

higher than that obtained from drill cores. Therefore, permeability values should

be used with caution for developing quantitative models of fluid flow.

7) Permeability from slug and pumping tests at thrust and reverse faults suggest

higher permeability values when compared to laboratory measurements. In both

cases, the permeabilites of the protolith and damage zone were found to be two

to five orders of magnitude higher than laboratory values.

43

8) Insufficient downhole data across fault zone exist in the literature for low-

permeability rock. Borehole-scale studies of fault properties and fault structure

are necessary to quantify in situ permeability structure of faults.

9) Although outcrop and core samples are often used to infer the permeability

structure of fault zones, it is clear that the spatial heterogeneity is a 3 D problem.

10) The majority of faults used to compile this report fall into the combined conduit-

barrier category of the Caine at al. (1996) classification. However, the

Carbonaras and Median Tectonic Line are more complex and do not fit within

Caine’s classification.

11) Scaling relationships between fault length and displacement, fault-core width

and displacement, and fault length and width exist, and should be taken in

consideration during regional site investigations. Doing so would provide

estimates for each fault component and possibly the extent of geological and

hydrological investigation.

12) The possibility exists that fractures are localized and focused on one side of the

fault, either in the hanging-wall or footwall. There is a few references related to

ore deposits suggesting focused mineralization on one side of the fault zone.

This would be a useful tool with which to classify fault-zone hydrology based on

fracture intensity and fluid flow.

2.9 Characterization Strategy

In this section, we describe some of the characterization strategies for classifying

fault-zone architecture used to minimize characterization efforts related to geological and

hydrogeological systems. Also, in analyzing the geology of the Japanese archipelago,

where faults are found ubiquitously, one of the main questions working with fault zones

in Japan is whether the fault is a long-lived inactive fault.

In the case of an inactive fault, several kinds of efforts could be useful:

44

(1) Analyses of rock types around the fault zone and the age of last reactivation

could help to estimate the extent of fluid-rock interaction (such as mineral

precipitation and alteration);

(2) Knowledge of the current stress field could provide insights into crustal

deformation/structural regimes.

(3) It is clear that in all types of rocks, faults are composed of two main

components: damage zone and fault core. Therefore, this classification is also

valid to site(s) in Japan.

(4) Scaling relationships can be used for fault characterization. Although there is

still a lack of consensus among scientists, there is enough evidence that assert

that linear relationships exist among fault length, displacement, damage zone,

and width of fault core.

(5) Investigators should test the hypothesis that some fluid flow is confined to one

side of fault zone, either on the hanging-wall or footwall. In this case, examples

of mineralized ore deposits would be valuable in providing evidence that certain

types of faults have such preferential flow characteristics.

(6) It is clear that there is a lack of in situ hydraulic investigation across/within

faults to understand their overall hydrologic effects to the surrounding flow field.

The majority of drill holes are focused on oil and ore exploration that treat faults

as boundary conditions, and groundwater management that concentrates on

shallow-depths studies.

(7) Based on our literature survey so far, the fault types (normal, reverse, and

strike-slip) by themselves do not appear to be a clear classifier of the hydrology

of fault zones. However, there still remains a possibility that other geologic

attributes and scaling relationships can be used to predict or bracket the range of

the hydrologic behavior of fault zones.

45

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3 断層の調査技術の実例、解析及びモデル化

本章では近年の各国各所での断層の現地調査の事例、原位置調査技術の実例、特に水

理性状に関する調査、そのデータを使ったモデル化や解析結果をまとめた。加えて異な

った水理性状を持った断層が存在する場合の揚水試験の数値シミュレーションを行い、

圧力変化の分布や経時変化を調べ、水理試験の解析の知見を得た。

3.1 諸外国での事例

3.1.1 アス、ブラジル

ブラジル北東部に存在するアス貯水池において、貯水量の増減による微小地震の観測

データを元に断層周辺のダメージゾーンの透水係数の空間的分布についての解析が行わ

れた。アクダムは 1983 年に先カンブリア時代の楯状地に建設された 34m の高さの土塁

ダムである。貯水池の水面の上下と微小地震の発生回数に相関性がある事が以前から知

られており、これは低い透水性のマトリックスに囲まれた高い透水性の断層内の間隙水

圧の拡散を通じて起こると考えられる。3 軸の高精度なデジタル地震計のネットワーク

を使って個々の断層内の微小地震の震源の複雑な分布と移動のパターンを解析する事に

よって、断層の水理パラメータの不均質性が存在する事が示された（do Nascimento et al.,

2005）。

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Figure 3-1 a)ブラジルアス貯水池 b) 微小地震と水深の関連性 c)水位変動と地震の遅れ（ do Nascimento et al., ）2005

3.1.2 アンデス南東域、ボリビア

ボリビアのサブアンデスゾーンの逆断層ゾーンのマクロとミクロ構造、及びセメント

分布の解析を行ったところ、2.5km 以深ではシリカが 80–90°C を超える温度により、圧

力による溶解-析出でセメンテーションが起こり石英が充填して閉じているが、浅部で

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は断層はオープンであり外来性の炭酸を含む地層水の側方流動の水みちとなっている可

能性がある。大きなスケールでは断層を横切る方向に低い透水性を示し、両側で異なっ

た過程の水が見られる(Labaume et al, 2000)。

Figure 3-2 断層の部位によって異なる炭素と酸素の同位体の分布(Labaume et al, 2000)

3.1.3 レッドシーヒルズ、エジプト

Sultan ら (2008)によれば、エジプトのレッドシーヒルズの断層水理の解析は以下に記

す数多くの手法（地質、物理探査、リモートセンシング等）を組み合わせて効率良く行

われた。(1) 地質図 (1:500,000 スケール) (2) ランドサットの TM（Thermatic Mapper）バ

ンド１~7 のリフレクタンスイメージ（解像度 28.5m）、（3）ランドサット TM バンド 2

（青）、4（緑）、7（赤）のフォールスカラーモザイク、（4）ランドサット TM のリ

フレクタンスバンド比モザイク(5/4 × 3/4, 5/1, 5/7)、（5）ASTER（Advanced Spaceborne

Thermal Emission and Reflection Radiometer）のレベル１A 生データ画像から作成された

数値標高モデル（DEMs, 解像度 60m）、（6）TOPAZ (Topographic Parameterization)手法

を使った DEM からの水系網と流域界マップ、（7）TRMM （Tropical Rainfall Measuring

Mission）の 1999-2001 年の累積降雨量データ、（8）以下の最低１つを含む孔井デー

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タ：位置、名称、井戸のタイプ、最大水位低下量、溶質成分、TDS、酸素と水素の同位

体比、及び地下水位。40 以上の地下水サンプル、EMWRI（Egyptian Ministry of Water

Resources and Irrigation）が集めた 20 年に及ぶ 180 本以上の井戸データ、(9) TM バンド 3

と 4 のリフレクタンスから作成した植生データ：NDVI （Normalized Difference

Vegetation Index）。これらのデータを総合した解析から、横ずれ断層は多くの場合、水

を蓄えたり水みちになっている事が判明した。

Figure 3-3 Red Sea Hills、Egypt の横ずれ断層と帯水層(Sultan et al., 2008)

3.1.4 北サマーセット、イギリス

イギリスの北サマーセット地方のジュラ紀の堆積岩に存在する小規模から中規模の大

きさの断層系のマッピングが Leckenby ら(2005)によって行われた。これらの断層は横ず

れのオーバーステップと正断層である。マッピングは異なった断層スタイル中の地下水

の流れの偏りや分布を予測計算する目的で行われた。方解石脈の厚さから過去の水の流

れの分布が推定された。フラクチャー密度、開口幅、コンダクタンスの 3 つのパラメー

タについて調査が行われた。その結果、フラクチャー密度と透水係数には相関性が殆ど

なく、フラクチャー密度は水理の不均質性を反映してないことがわかった。反対に開口

幅とコンダクタンスは水の流れの偏りを反映すると結論された。断層タイプによって違

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った特性があり不均質性のパターンが予測可能であった。正断層系では、リレーランプ

ではなく、断層沿いに最大の水の流れの局所化がおこり、収縮横ずれ断層では断層内に、

膨張横ずれ断層ではオーバーステップに強い局所化が起こっている。しかし、何れのケ

ースもオーバーステップのダメージの度合いが不均質性のパターンを左右している。

Figure 3-4 横ずれ断層沿いの不均質性の偏り b),f):割れ目頻度、c),g)開口幅、d),h) コンダク

タンス分布。Leckenby et al, (2005)

3.1.5 オランダ

Bense と Kooi(2004)は空間的に密に(<10m)、季節毎に数回計測された浅部の温度分布

データを使って垂直方向の地下水流動の水平分布を推定した。現場はオランダ南東部の

未固結の珪質砕屑堆積岩を切る Peel Boundary 断層ゾーンである。Peel Boundary 断層は

この地域で各所で水平方向の流れに対して不透水性を示す重要な断層である。2℃以上

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の温度異常が近在しており、季節ごとに逆の異常を呈する。Bense と Kooi は水と熱の流

れの数値モデルを使って地下の温度の時間的、空間的な変動が地表の季節の温度変化と

地下水の流れの分布との相互作用に起因していることを示した。温度の水平分布に加え

て、井戸の深度方向の分布を制約として使って大きなスケールの地下水の流れの仕組み

を推定した。地下の温度パターンをシミュレーションするには長期にわたる地表の温度

変化も考慮することが必要であった。彼らの研究は近年の気候変化の影響を組み入れた

点で新しい。

Figure 3-5 断層ゾーンPeel Boundary の水頭分布、涵養量と湧水量分布（Bense とKooi,2004)

Bense と van Balen(2003)は Roer Valley Rift System (RVRS)の断層が地下水流動に及ぼ

す影響を露頭スケールから広域スケールのデータを使って解明した。RVRS の断層は水

平方向の流れに対して強い不透水性を示す一方、同じ位置で垂直方向の流れの水みちが

卓越する。断層が横切る堆積岩の地下水流動をモデル化する際にはこの強い非等方性を

考慮する必要があると説いた。Geleen 断層ゾーンの露頭では粒子の流れと粘土スミアに

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ついて細かく調査することによって変形メカニズムが解明できたとしている。断層中の

ミクロスケールの水理性状の分布を定性、定量的に推定した。加えてコアプラグの計測

からダメージゾーンが優先的な水みちを形成する可能性が示された。より大きなスケー

ルでは Peel Boundary 断層は Uden 村の断層の近傍では断層の垂直方向の水みちが卓越し

て地下水が上昇して湧出していることが確認される。断層の存在とその水理性状の分布

が如何に広域スケールでの地下水流動パターンに大きな影響を与えているかの例として、

ドイツのリグナイト鉱山の近辺の水頭分布の調査の事例を示している。

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3.1.6 ベルギー

Bense と van Balen (2004)によれば、ベルギーの Lower Rhine Embayment の未固結堆積

層では断層が地下水流に大きな影響を及ぼしている。この地域の地下水頭マップ.を解析

すると、多数に存在する個々の断層は地下水の流れに対してして難透水性であるが、断

層のリレー構造が帯水層どうしを水理的に繋げる役割をしている。Lower Rhine

Embayment にある最大規模の露天掘り鉱山の近くの Rurrand 断層の周りの地下水流動パ

ターンは断層リレーゾーンを廻る流動の代表的な例であり、多くの記述が残されている。

Bense らは Shale Gouge Ratio (SGR)を使って Cleay Smearing の影響を定量的に計算し、断

層リレーの水理も数値モデルを使って解析した。その結果、帯水盆において、断層の封

水程度を推定する際に断層リレーの影響を無視できないことがわかった。同時に水理挙

動予測モデルにおいて、SGR は断層の水理性状の推定に有望な手段であることを示した。

Figure 3-6 砂と粘土の互層を切る断層沿いの Shale Gouge Ratio (Bense and van Balen, 2004)

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3.1.7 ブリタニー、フランス

Borgne ら(2006)は孔間フローメーター試験を使ってフランス、ブリタニーの沿岸付近

の正断層に付随した割れ目の卓越した結晶岩性 Plœmeur 帯水層内の水みちの連続性に関

する特性評価を行った。その結果、高い透水性が少なくとも 150m 以上サイト全体で連

結している事が判明した。大きなスケールではボアホールスケールの不均質性は消滅し、

高い透水係数側に収束する事が示された。地下の流れの場の構成、即ち高透水ゾーンの

連続性に起因していると考えられる。

Figure 3-7 Plœmeur帯水層を切る正断層と高透水ゾーン(Borgne et al, 2006)

3.1.8 プロバンス、南東フランス

地震災害予知の目的で活断層の特定とオフセット量や再発度などの地震変形パラメー

ターを限定する為に浅部の物理探査手法を用いた調査が頻繁に行われるようになった。

Nguyen ら(2007)はフランス南東部のプロバンス地方で電気トモグラフィーを使ってゆっ

くり動く逆断層の場所の特定と複雑な地質環境下での電気トモグラフィーの断層探知に

関する有用性の検証を行った。これは詳細な地質と地形の調査を組み合わせることによ

って初めて可能であった。褶曲した泥灰岩と石灰岩の互層した地層では電気探査イメー

ジはスケールによっては電気伝導度のマクロな非等方性に大きく左右される。特に解像

度が層厚より大きい場合は逆解析によって構造を解明することが困難である。順解析モ

デル中の地質構造のデータを計測データと逆解析結果を比較しながら試行錯誤的にアッ

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プデートしている。浅部では解像度が非等方性のスケールより同じか小さい為、電気探

査トモグラムによって比較的簡単に地下構造の解明が可能であった。

Figure 3-8 らNguyen (2007)が電気トモグラフィーを行った南フランスの断層

3.1.9 コリントス湾、ギリシャ

Micarelli らはギリシャのコリントス湾に存在する活断層のフラクチャーネットワーク

と断層水理について研究を行った。この正断層の両側で、少なくとも限られた時間と箇

所で圧力の違いがあり、断層を横切る方向に遮水性があると考えられる。断層の上盤と

下盤の両側の炭酸塩セメントを解析した結果、断層の発達の全段階に渡って局所的に遮

水的であったと判断される。しかしながら、一方、井戸から得られた水のサンプルや圧

力から推察すると、高地から海岸にに断層ゾーンを横切って天水が涵養していると考え

られる。現地の定量的調査とコアの実験から広域スケールから断層のフラクチャーネッ

トワークまで特定が可能であった。さらに、薄片の解析によって断層を形成する岩の種

類まで特定した。これらのデータから断層の透水性を評価した。簡単な 3 次元の流動モ

デルを使い、断層の透水性構造、堆積物、地下水流動、断層の垂直方向のずれ、天水の

涵養などをモデル化している。その結果、断層はセグメントごとに遮水性があるが、リ

レーの部分がに透水性があり、Peloponnese の高地から海岸までの広域流動を可能にして

いると結論した。

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Figure 3-9 Micarelli ら(2006) の提唱する正断層の発達過程と水理性状

3.1.10 コリントス、ギリシャ

Doan ら(2007)はギリシャのコリントス山脈にある Aigio 活断層を貫く 1000ｍの深さの

ボーリング孔の温度分布を測定した。熱流量は 53 mW/m2 であり、造山活動が与える影

響は殆ど無いと結論された。断層の近傍の温度は純粋に熱伝導のみの効果を考慮したモ

デルより高い。孔内圧力の長期にわたる測定結果から、これは断層の上側の地下水の流

れに起因するとは考えにくい。さらに、Aigio 断層は小さな正断層で近年に動いていな

いことから、断層が滑った際の熱が原因であるとは考えにくい。これらのことから、今

度異常は断層の下盤のカルスト地層内の熱対流が原因であると推察された。数値モデル

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による計算でも測定値に近い高い温度値となった。これは地熱地帯以外の地域で断層付

近の対流によって温度異常が起こされている珍しいケースと言える。

3.1.11 SOBREDA, 北ポルトガル

Antonio と Pacheco(2002)は傾斜の緩い逆断層で多数の短期可変揚水量試験を行った。

この断層は衝上断層が多く存在する北ポルトガルの Sobreda 地域に位置し、蛇紋岩を横

切っている。断層ゾーンは地質マッピング航空写真、および、電磁気探査によって存在

と形状が特定された。ボアホール試験の解析の結果、断層の透水係数は 7×10-7～2×10-

4m2/s、貯留係数は 2×10-5～6.1×10-4 と推測された。

Figure 3-10 Aigio 断層の断面図と断層を貫くボアホール沿いの温度分布(Doan ら、2007)

67

Figure 3-11 ゆるい傾斜の逆断層の可変揚水量試験 ( とAntonio Pacheco, 2002)

3.1.12 タグスバレー、ポルトガル

Carvalho らはポルトガルタグスバレー下部に位置する多くの伏在断層を特定する為に

は間接的な手法に頼らざるを得ないとしている。彼らは高精度の反射波を使った地震探

査手法を使って近年の堆積物によって隠されたテクトニック構造の浅部の断層の特定に

成功した。もっともこれらの断層は石油会社による深部の反射法による震探と地表の地

質調査から存在が推定されていた。WNW–ESE-方向の断層帯(Porto Alto fault)と NNE–

SSW-方向の逆断層帯(Vila Franca de Xira–Lisbon fault).である。高精度の反射法と垂直電

気探査法（Vertical electric sounding, VES）、さらに屈折波を反射法データから解析する

方法を組み合わせることによって断層の特定が可能になった。

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Figure 3-12 高精度の反射波を使った地震探査によって特定された断層群(Carvalho et al., 2006)

3.1.13 LAC DU BONNET, CANADA

カナディアンシールドでは断層とフラクチャーが主要な地質構造であり、花崗岩中の

水の流れをコントロールしている (AECL, 1994a, 1994b)。Lac du Bonnet の URL

（Underground Rock Laboratory）では、貫入岩中の応力場から断層やフラクチャーの透

水性を推測し、地下構造物の長期的な安定性の予測に利用した。以下、カナダの Lac du

Bonnet の URL の概要調査段階で断層の検知や性状の特定に有効であった物理探査手法

をリストする。

• 航空電磁気測量

• VLF-EM

• 航空磁気測量

• 重力探査

• サイドスキャンレーダー

• 地表電気探査

• 反射法地震探査

3.1.14 ポンジョンギ、韓国

地下水組成の解析から断層の水理性状が推定された１つの実例として、韓国南東部の

パンジョンギ地区での事例が挙げられる。当該地域では 10％の飲料用井戸（422 本）の

69

水が環境基準（1.5mg/l）を超えるフッ素を含有量している原因を調査する目的で自然同

位体等の地化学データを使った研究が行われた。その結果、基準を超える地下水を産す

る井戸の大多数が断層沿いに存在することが判明した。安定同位体を使った解析により

地下水の相対的な年代特定が可能となり、フッ素含有量の高い地下水は深部で循環して

いる古地下水が断層沿いに上昇していると結論付けられた。しかしながら、断層に付随

した亀裂が発達している断層ゾーンではフッ素濃度の高い地下水は殆ど観測されていな

い。これは同位体解析によれば逆に若い涵養水が断層沿いに深部に入り込んででいると

推測される。

Figure 3-13 断層の垂直方向の高透水性による地下水のフッ素含有量の相違(Kim and Young Jeong, 2005)

3.1.15 台湾

水の拡散係数が水圧変化を左右することにより、破断の際の有効垂直応力を支配する。

1999 年のマグニチュード 7.6 のチチ地震のモデル計算によると断層が十分にシールされ

ていたとすれば、大きなずれが起きた箇所で断層内の水圧が高まったことにより高周波

の揺れが抑制される現象の好例であることが示された。Doan ら(2006)はこれらのモデル

の鍵となるパラメーターである水の拡散係数を干渉井を使って原位置で計測した。その

結果地震の際の Chelungpu 断層で予想される値に矛盾しない D = (7 ± 1) ×10-5 m2/s とい

う低い値を得た。殆どの多孔質弾性体では水の貯留係数 S は 10-7から 10-5の間の値であ

ることを鑑みると断層沿いの水の透過率 T は 10-11から 10-9m2/s であることになる。それ

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に対応する透水係数は 10-18－10-16 m2/s となり、母岩のコアから計測された透水係数の最

大 100 倍大きい。すると断層ゾーンの圧力は 0.06 から 6MPa 加圧されたことになり、こ

れは地盤圧力の 0.2%から 20%と計算される。

Figure 3-14 らDoan (2006)による Chelungpu 断層の水の拡散率の測定の際のボアホー

ル配置

3.1.16 FAIRBANKS, ALASKA

熱分解ガスクロマトグラフィー /質量分析 (py-GC/MS) は地下水中の溶存有機物

（DOM）を使った地下水の起源の指紋捜査の手段として有効である。White ら(2002)は

Py-GC/MS 分析を使った実例として、アラスカ州フェアバンクスの永久凍土に覆われた

複雑な地下水理構造を解明した。彼らは２つの分水界地域の関連性を調べる為に地下水、

湧き水、小川の水などのサンプルの指紋捜査的分析を行った結果、分水嶺の反対側同士

の湧き水は同じ水源であることが明らかになったとしている。また、DOM の指紋的特

徴のみを使って地下水の起源や種類の分類が可能であると説いた。これにより、DOM

解析が複雑な分水界地域の水みちの解明や汚染物質の移行に関する役割といった DOM

の重要な特性の評価に繋がる可能性ができた。

71

3.1.17 BLUE RIDGE PROVINCE, VIRGINIA

Seaton と Burbey (2004)はバージニア州の Blue Rige Province の地下 300m 以深にある断

層は大きな貯留能力を持ち、多大な量の地下水を井戸に供給することが可能だと説いた。

さらに、これらの断層は深部の帯水層をコンパートメントに仕切っていると考えられる。

深部の帯水層への水の涵養と帯水層からの流出は上部の封圧層からゆっくり漏れるか、

あるいは過去の変成期に高い濃度のクォーツが生成し、後の変形エピソードに割れ目が

発達した局所的な裂け目ゾーンを通って起こると考えれる。彼らはこの地域の逆断層の

重要性を説き、Blue Ridge Province の水理モデルに古い断層構造を取り入れることを提

唱した。結晶岩環境の中の断層は周辺の水理に影響を及ぼすと言うより、地域の水理特

性を支配すると述べている。

Figure 3-15 Blue Ridge Province の逆断層の高い貯留能力(Seaton and Burbey, 2004)

72

3.1.18 ルイジアナ州

Bense と Person(2006)は珪質砕屑堆積岩の帯水層で頻繁に観測される断層ゾーンの高

透水性と難透水性の二面性は断層が水理学的に極端な非等方構造を呈すると解釈する事

が可能であると説いた。断層の水理学的非等方性は粘土スミア、砂の引きずり、粒子の

再配列、断層の垂直方向の分断など種々のメカニズムが成因となる。Bense と Person は

断層ゾーンの幅、岩相の不均質性、水理的非等方性を予測するアルゴリズムを提唱した。

73

Figure 3-16 モデル化した 3 種類の断層を横切る流れと物質移行（Bense and Person, 2006）

74

3.1.19 アルボード、オレゴン、米国

Heffner と Fairley (2006)は米国オレゴン州のアルボード盆地の断層の水理構造の研究

を行い、断層は空間的、時間的に不均質であり、断層内を流れる水に対して高透水性、

低透水性、あるいはその両方の性状を同時に呈することを示した。彼らは正断層に沿っ

た 702 箇所の泉や地表面で測定された温度分布を基に地球統計学的手法を使って、活動

的正断層の透水係数の分布のモデルを構築した。断層面に平行する方向の流れは広い範

囲で低い透水性から中程度の透水性を示すが、数多くの個別の流れの水みちが存在する

ことを発見した。この結果は結晶岩や強く岩石化した堆積岩の断層の概念モデルとは良

く一致するが、アルボード盆地のような沖積層や軽度に岩石化した堆積岩内の断層に予

測される性状と大きく異なり、一層の研究の必要性を説いている。

.

Figure 3-17 断層沿いの湧き水の温度分布と地球統計学手法を使ったシミュレーション

(Heffner and Fairley, 2006)

また、Fairley と Hinds(2004a, 2004b)は断層のステップオーバー付近では複雑な 3 次元

的な流れの循環挙動を示すことを発見した。こういった現象は正確な断層付近の流れの

概念モデルや数値モデルを構築する際に非常に重要であると説いている。

75

Figure 3-18 断層のステップオーバー内の地下水の 3 次元的流れ(Fairley and Hinds2004a, 2004b)

3.1.20 ワシャキエ山脈、ワイオミング州

Evans ら(1997)は花崗岩中の逆断層ゾーンの露頭の各部分、各方向から岩石のコアを

採取し、応力下および非応力下で透水性の計測を行った。その結果、少なくとも岩石コ

アのスケールでは断層のダメージゾーンは母岩と断層コア部の間に位置する透水性の高

い部位であることが判明した。ダメージゾーンのコアは応力下でも母岩やコア部のコア

に比べて透水係数の減少度が低い。これらの結果は花崗岩に関する過去の原位置での水

の流れの観測結果と一致するとしている。ダメージゾーンとコア部の 2 つの部分からな

る構造は断層全体としての水の流れに対して非等方性を形成する。ダメージゾーンの発

達した断層ゾーンは断層面に平行で比較的薄い平板状の部分で流れが卓越する。一方、

断層コア部は断層を横切る方向の流れに対して透水性が低い。

76

Figure 3-19 らEvans (1997)の採取した逆断層ゾーン内のコアサンプルの位置と方向

3.1.21 ハンナ盆地、ワイオミング州

Johnson と Huntoon(1994)によれば、ワイオミング州のハンナ盆地のシャーリー逆断層

の上盤沿いの地下水の流れは概ね地層に沿っている。異なる帯水層間の層序を横切る地

下水の流れは加圧層や透水層を分断する正断層や亀裂を伝ってのみ流れる。こうった層

序に平行、垂直な構造が高地の涵養域から流出域までの水みちの構造を形成する。シャ

ーリー山脈のマジソン層から涵養された水は水頭の低い方向へ流れるが、Tensleep 帯水

層に上昇し、低地に達する前に正断層を伝ってハンナ盆地の上辺付近の Tensleep 砂岩が

露頭する辺りで地表に流出する。

77

Figure 3-20 シャーリー逆断層周辺の地下水の流れ(Johnson and Huntoon, 1994)

3.1.22 リオグランデリフト、ニューメキシコ州

Mailloux ら(1999)は断層ブロックのテクトニックによる動きと断層の透水構造がニュ

ーメキシコ州ソッコーロ市に近いリオグランデリフトの過去と現在の水理地質に及ぼす

影響を数値モデルを使って解析した。La Jencia と Soccoro 盆地の過去と現在の地下水の

流れのパターンと温泉形成の解明を目的としている。数値モデル解析結果と過去現在の

流れのデータが最も一致したのは断層の水理性状を高透水性でも低透水性でもないと仮

定し、2km の長さの亀裂性の結晶岩に中程度の透水性を持たせたケースであった。定性

的には地下水盆の中の地下水の流れの変化は主として帯水層や封圧層が途切れたり、つ

ながったりすることに因ってもたらされる。山脈フロントの涵養水の約 5％が堆積岩下

2.8km の深度まで浸入していると考えられた。これまでは地下水源としてリオグランデ

リフト谷の浅部の堆積層のみ対象としていたが、新たな地下水源として期待できる可能

性がある。

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Figure 3-21 リオグランデリフトの断層ブロック沿いの地下水の流れ (Maillouxら, 1999)

3.1.23 ロスアラモス、ニューメキシコ

ロスアラモス地域の地下水の涵養の物理的プロセスは未だに解明されていない。特に、

Pajarito 断層ゾーンが地下水の涵養に与える影響（水みちまたは反対の障壁なのか）に

関して殆ど判っていない。断層水理に関するデータが殆ど存在しない為に地域の地下水

モデルも不完全と言わざるを得ないと Dale ら(2005)は述べている。Dale らは 2000 年 5

月に起きた Cerro Grande 山火事によって地下水にもたらされたトレーサーを使って断層

の露頭（PA-106）および下流の湧水点の地化学的組成を比較し、涵養水の源を推定する

ことが可能であった。山火事とその夏のモンスーンによる洪水によって、PA-106 に於

いて HCO3, SO4, Ca, K, Mg, Mn, Sr,さらに TDS が 4 倍に膨らんだ。その 1 ヶ月以内に

Homestead と Starmer Springs に於いて同様の痕跡が確認されたことから、Pajarito 断層ゾ

ーンが地下水の涵養に重要な役割を担っている事が推定される。上記以外に解析に使っ

た地化学成分は 3H, 4He, Ar, Ne, δD, δ18O であった。

また、Howley(2005)によれば Estancia 流域に全体於いて小流域を区切る断層に伴う亀

裂と溶解プロセスによってもたらされた間隙率と透水係数は基盤岩中の地下水の流動と

特質に明白に影響をもたらす。Koning(2005)は同地域において、航空磁気測量のリニア

メントが断層の特定に役立つことを示した。多くの場合航空磁気測量データとマッピン

79

グの結果が一致して理う。これらの断層が地下水流動と帯水層のコンパートメント化に

対して絶大な影響を及ぼしていることは疑いがないとしている。

Rodrigues and Williams は 16 の MT ステーションを使って北西の Pajarito 断層と南東の

La Bajada 断層で区切られる La Bajada 収縮内の地下構造を探った。MT 測量はそれぞれ

の年代の岩石に予測される電気抵抗値を元に基盤岩の相対位置と古生代‐中生代‐第三

期の堆積岩の厚さを推定した。例えば粘土鉱物の少ない、水で飽和した第三期の堆積岩

は比較的低い抵抗値(10 to 50 ohm-m)であり、一方、中生代の頁岩は 1 から 5 ohm-m とい

った非常に低い値を示す。地表近くの第三期の玄武岩、安山岩と流紋火山岩は非常に高

い 100 から 5,000 ohm-m の抵抗値を示し、埋没した古生代の炭酸塩堆積物は 200 から

500 ohm-m である。

3.1.24 EAST TINTIC MOUNTAINS ユタ州

Hamaker(2005)は Eureka Lilly 断層ゾーンは East Tintic 鉱山地区の地下水流に対して不

透水バリアとなっていると結論した。断層の両側の地下水が異なった温度、組成、さら

に水頭圧であるというのが根拠である。断層のダメージゾーンは開口割れ目の卓越した

連結したネットワークを形成しており、断層の交差する箇所は水みちとなっている。断

層のコアブレッチアは再セメント化し、鉱化して空隙を埋めていることにより、厚い不

透水ゾーンを形成している。これにより、断層が両側の地下水の仕切りとなっている。

このコンパートメント化により、従来の流域モデルのコンセプトが不正確で当てはまら

ないと結論した。

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Figure 3-22 不透水バリアとなっている正断層の両側の水頭と水温(Hamaker, 2005)

3.1.25 ハリケーン、ユタ州

ユタ州、ハリケーン市近くの Timpoweap Canyon にハリケーン断層コア部とダメージ

ゾーンの理想的とも言える露頭が存在している。さらにこの谷ではダメージゾーンの各

所から湧水してバージン川に流れ込んでおり、断層ゾーンからの湧き水の研究に最適で

ある。ハリケーン断層は急傾斜の活動正断層で、長さが 250km、ずれが 2500m である。

Timpoweap Canyon のダメージゾーンの亀裂性石灰岩からは~40°C の温水が 260 L/s、二

酸化炭素が 4 L/s の流量で湧き出ている。また、湧き水中のδ2H と δ16O の組成から地下

水は約 3km 深の基盤岩にから出ていると考えられる。一般の断層ではダメージゾーンの

亀裂密度は断層から遠くなるにつれ減少する傾向にあり、湧水量もそれに伴って減ると

考えられているが、Timpoweap Canyon のダメージゾーンはその法則に従わない。これ

は Laramide と Sevier 造山運動の応力によって形成された既存の亀裂が原因だと考えら

れる。ジプサムの溶解による陥没構造と大きな亀裂が湧水箇所を決定している (Dutson,

2005)。

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Figure 3-23 ユタ州 Timpoweap Canyon のハリケーン断層の断面図(Dutson, 2005)

3.1.26 BIG HOLE FAULT、ユタ州

Shipton ら(2002)は露頭と 4km の長さの Big Hole 正断層を貫く 5 本のボーリング孔(数

十ｍスケール)から断層の構造と透水係数の分布を調べた。Big Hole 断層はユタ州中央部

のジュラ紀の Navajo 砂岩に露頭している。ずれが 8m と 3~5ｍの 2 箇所にサイトから合

計 363.2 m のコアを採取した。断層コア部は最大 30cm の幅で著しく粉砕された空隙率

の低い融合された変形帯で片側、または両側にすべり面が確認される。Probe

permeameter を使った計測では断層に近づくにつれ、2000md から 0.1md に減少した。断

層コア部のコアを使った計測ではコア部は 1md 以下の透水係数であった。これらのデー

タを使い、Shipton らは断層のバルクの透水係数を計算した。それによれば、5m~10m の

スケールでは 30-40md で、母岩の 1~4%の透水係数であった。複数の断層が複雑に折り

重なったケースを想定した場合 7–57 md であった。断層を横切る透水係数は断層のパラ

メーターの内、一番大きなばらつきを示す断層コア部の厚さに最も大きく左右されるこ

とが明らかになった。

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Figure 3-24 断層の厚さと断層のバルクの透水係数の関係(Shiptonら、2002)

3.1.27 エルクホーン、コロラド州

Marler と Ge (2007)はコロラド州 South Park 市の Elkhorn 断層の水理性状と断層が広域

水理に及ぼす影響を調べた。Elkhorn 断層は傾斜のゆるい逆断層で先カンブリア時代の

結晶岩が South Park Basin の堆積層に押し上げられている。Elkhorn 断層は断層を横切る

方向の透水性が低く、結晶岩の帯水層と South Park Basin の堆積層の地下水を分断して

いると考えられている。Marler と Ge は断層の水理性状を特定する為にボアホール試験

に加えて、直流電気抵抗試験を行い、断層の不均質性の構造を調査した。さらに、数値

モデルを使って地下水の流れのシミュレーションを行った。その結果、断層は 3m の厚

さの低透水のコア部と高透水性のダメージゾーンで形成され、断層を横切る方向の流れ

は極めて制限されていることが判明した。

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Figure 3-25 ユタ州エルクホーン断層の電気抵抗トモグラフィー解析結果 (MarlerとGe, 2007)

3.1.28 MEDINA, テキサス

Clark と Journey (2006)はテキサスの Medina 郡北部のリレーランプとそれらに関連する

断層が地下水の流れを 4 つの個別の流れの経路に分けて南西方向に流していると説いた。

Medina の北西の流路は北側に Woodard Cave 断層が在り、南は Parkers Creek 断層が存在

する。地下水は Seco Creek 沿いの直交する断層に突き当たるまで地層の傾斜方向である

南西に向かって流れる。その後この断層が難透水性である為、一部あるいは殆どの地下

水を南向きに流す。

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Figure 3-26 テキサス州 Medina 群の断層にコントロールされた 4 つの地下水流(Clark and Journey, 2006)

3.1.29 HIDDEN VALLEY FAULT, TEXAS

SWRI は近年洪水により露出したテキサスの南部中央に存在する白亜紀の Glen Rose

地層を切る Hidden Valley 断層を使って断層の性状評価のプロジェクトを立ち上げた。

Hidden Valley 断層は炭化水素の油田にありがちな代表的構造であり、このサイトは断層

構造、岩種と断層ゾーンの水理性状の相関性を調査できる類のないサイトである。当該

サイトでは以下のような研究計画が立てられている。

1. 層序学的、岩相学的特性評価

上盤と下盤の詳細な断面マップの製作自然ガンマ線計測ログ母岩の岩相、表面性状と粘土含有量の定量化

2. 地質構造の解析

層序の力学的な解析露頭スケールの構造的要因の詳細なマップ

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微細構造解析地質マッピングとボアホールデータからデジタル地質構造モデルを構築

3. ボアホール関連

掘削、コア採取、検層の後、上盤側と下盤側の多区間に水圧計を設置

4. 水理学的特性評価

Canyon 湖と近辺の井戸の水位データの収集地表の泉、地表水の浸出点、侵入点のモニタリング Canyon Lake 渓谷の流水量測定観測井内の多区間での水圧モニタリング

5. 力学的特性評価

断層の露頭部の岩の力学的性状の解析、試験有限要素法を使った Hidden Valley 断層の応力変形解析

Figure 3-27 テキサス州、Hidden Vally 断層の露頭トレース（SWRI, 2008）

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3.1.30 YUUCA MOUNTAIN、ネバダ

Bredehoeft (1997)はネバダ州ヤッカマウンテン付近の地下水面の温度の分布からヤッ

カマウンテンの東側と西側の両側の主だった断層沿いに地下水が深部から上昇して来て

いると推察した。Bredehoeft はこれらの断層の透水係数が非常に重要であるにも拘らず

データが不足していると述べている。

Figure 3-28 ヤッカマウンテンの断層群(Bredehoeft, 1997)

Blakely ら (2000)は地下の複雑な地質構造を探る目的でヤッカマウンテンの南の

Amargosa 砂漠とその付近で高精度の航空磁気測量を行った。測量は面積にしておよそ

7,700 km2 (2,970 mi2)をカバーし、ネバダ州 Beaty からカリフォルニア州 Shoshone までの

Nevada 核実験場とデスバレー国立公園を含む広い範囲であった。航空磁気測量の測線は

東西に 400m 間隔で高度 150m、もしくは安全とされるぎりぎりの高度で飛行した。特徴

的な磁気異常は Yucca Mountain や Greenwater Range などの火山岩地形と Bare Mountain

や Black Mountains.のような原生代基盤岩で観測される。火山岩の断層によるずれが磁

気異常のリニアメントの原因となり、浅い断層の詳細なマッピングが可能となる。特に

興味深いのは Devils Hole と Pahrump Valley に見られる堆積層での磁気異常である。泉

の存在箇所が磁気異常のリニアメントと一致することから、これらは断層が深部の磁性

岩をずらし、堆積層も切っており、地下水流動に大きな影響を与えていると考えられる。

Funeral Mountains 上の磁気異常のリニアメントは一般に非磁性の Stirling 珪岩を切ってい

る先カンブリア時代の北-北東方向の走行の断層群と一致する。これらの磁気異常の箇

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所と方向を Furnace Creek の北に位置する湧水箇所と比較して考えると、断層は北から

Death Valley 方向へは水みちとなっていると考えられる。しかし、この磁気異常の鉱物

学的な説明はついていない。

Figure 3-29 ヤッカマウンテン南西の断層による磁気異常のリニアメント(Blakely ら、2000)

Flint ら(2001)のヤッカマウンテンの概念モデルでは断層は地表から処分場レベルを通

って地下水面までの水の流れの直線的な経路になっているとした。未固結の Paint Brush

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(PTn)と呼ばれる凝灰岩地層そのものは難透水性であり、天水は傾斜にそってほぼ平行

に流れるが、断層が PTn 地層を切っている部分で断層に入り、地下水面への最短路であ

る断層を伝わって飽和領域へと流れる。処分場レベル以深では断層が地下水の流れと物

質移行を支配してる。

Figure 3-30 Flint ら(2001)のヤッカマウンテンの不飽和部の概念モデル。断層が早い水みち

となっている。

IAEA(2002)はヤッカマウンテンの飽和領域の特性評価のレビューにおいて地下水の流

れに対する断層の役割が未だに不明であり、その結果重要な地下水の流動経路に関して

未解明である事に大きな懸念を示した。

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Figure 3-31 ヤッカマウンテン周辺の主要断層とトレンチの位置 (IAEA, 2002)

Simmons ら(2004)によれば、地表からの概要調査の為の物理探査手法を用いて断層の

存在、地層の分布、さらに埋没している火山の形や大きさ(Ponce, 1996; Sikora et al.,

1995)に関する情報を得た。ヤッカマウンテンサイトで第一義的に解決するべく努力す

べき問題は処分場ブロック中の断層の数、スタイル、深さと連続性の解明である為、

種々の物理探査の手法を用い、どの手法が最も断層のイメージの取得に有効か比較検討

された。その結果、どの１つも有効と言えず、正確な地下構造の情報を把握する為には、

いろいろな手法を組み合わせる必要があることが明らかになった。

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重力探査や磁気探査のような広域の物理探査データは火山岩の構造、異なった岩相の

コンタクトと断層のオフセットの同定に使われた(Langenheim, 2000c)。地震探査の屈折

波（S 波と P 波の速度）は上部地殻、ヤッカマウンテンとその周辺の速度構造をモデル

化する為に使われた(Smith ら, 2000d)。近年行われた航空磁気測量は地域の多くの埋没し

た火山の大きさと形の同定に使われ、ヤッカマウンテン周辺の火山災害の予知に役立っ

た(Smith and Keenan, 2005)。物理探査は少なくとも数十メートルのオフセットのある断

層の検知に役立ったが、断層の存在の検証には複数の手法の組み合わせが必要となった。

埋没した断層とその他の地質の不均質構造を検知する為にマグネトテルリック法（MT

法）を試みたが、他の手法と組み合わせなければあまり有用でないことが判った。一般

的な結論としては、断層を検知するにはスタンダードな物理探査手法を用いた場合、断

層のオフセットが少なくとも数十メートルはある必要がある(Simmons ら, 2004 Section

4.6.5.3).。

ヤッカマウンテンサイトに於いて、断層の水理データはあまり取得されていない。透

気性、間隙率やトレーサーの移行のデータを取得する為に空気のインジェクション試験

やトレーサー試験が Ghost Dance 断層や Bow Ridge 断層で行われた(BSC, 2004b, Section

2.2.3)程度である。断層は地下水の流れに対して、主要な水みちになったり、横切る方

向に不透水性であったりし、その結果、宙水層を形成したりする(Flint ら., 2001)。

岩石や鉱物の地化学性状は主として異なった岩相沿いの岩と水の相互作用と断層やフ

ラクチャー沿いに析出する二次鉱物に影響を受ける。これらの相互作用は岩石鉱物の溶

解反応、イオン交換反応、加水分解反応、析出反応、酸化反応その他の変化や反応であ

る。断層の鉱物学的性状は水と物質の移行の評価に重要である。断層沿いの鉱化が皆無

であったり、希少であれば断層は高透水性である可能性が高い。以下、ヤッカマウンテ

ンでの断層の検知や特定に有効であった手法を列挙する。

• 衛星イメージデータ(レーダー、スペクトル解析等)

• 異なったスケールの物理探査

• 広域、地表地質調査

• リニアメントマッピング

• 岩石サンプルの取得

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• 間隙水や宙水中の地化学データおよび同位体の痕跡（Cl, 3H 濃度）と充填

鉱物

• トレーサーインジェクションテスト

此処で特筆すべきは、上記リストの最後のトレーサーテストは我が国の概要調査では

全く実施すべきでないと考える。ヤッカマウンテンでのトレーサー試験は候補地が１つ

に絞られた後の精密調査段階で行われたものであり、極めて小さいスケールの調査であ

った。

3.1.31 NEVADA

Hammond と Evans(2002)は Keno 断層ゾーンの地化学的痕跡と断層構造が鉱石鉱化成

分を含んだ流体の流れに対する影響、及び露頭している鉱石を使って正断層の構造と成

因に関連するプロセスの調査を行った。当該断層は北部ネバダ州の中央付近に位置し、

シルト岩およびシルトを含んだ石灰岩中のる 55m から 100m の傾斜方向にずれた、傾斜

の緩やかな正断層である。走行を横切る方向に 180m、傾斜方向に 65m、走行方向に

350m の露頭があり、岩相の混在する炭酸塩堆積岩で、断層、フラクチャーと地下水が

如何に相互作用してるかの研究に理想的なサイトである。断層の上盤も下盤もフラクチ

ャーが極めて多いが、フラクチャーの密度は断層からの距離によってさほど変化してい

ない。断層のずれは 100%傾斜方向のずれから 100%走行方向のずれに変化している。ま

た、1m から数十 m のスケールでは鉱物化のパターンとフラクチャーの密度に相関性が

見られない。断層は高透水性と難透水性を併せ持っていると考えられ、断層は断層が形

成された時期に発達したフラクチャーと鉱物を含んだ水をつなげる役割を果たしたと考

えられる。その後断層は局所的に炭化水素を含むカルサイトの鉱脈を形成する水みちと

なった。断層の性状は短距離、鉱床スケール、貯留層スケールの距離で激変する。大き

なスケールでは可能性があるが、詳細スケールではダメージゾーンの存在と断層の地化

学的区分とは相関性が無いことが判明した。

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Figure 3-32 ネバダ州 Keno 断層ゾーンとスキャンライン(Hammond and Evans, 2002)

3.1.32 VALLEY OF FIRE, NEVADA

Jourde ら(2002)は詳細なスケールの現地調査データを使って大きなスケールの横ずれ

断層の等価な透水係数を計算した。彼らは既存のネバダ州の Valley of Fire 州立公園内の

断層の特性評価データと個々のフィーチャーの性状データを利用した。当該断層(複数)

は亀裂ゾーンの横ずれによって形成され、粒子の細かい断層岩（ガウジ）と変形バンド

と周辺のジョイントとずれジョイントの発達したダメージゾーンから成る。詳細な断層

ゾーンのマッピングと実データにキャリブレートされたイメージデータに基づく透水係

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数を使って差分法を使って断層ゾーンの数値モデルを構築した。ずれの長さがそれぞれ

6m、14m、150m のケースの断層をモデル化した。何れの場合も断層ゾーンの透水係数

は強い非等方性を示した。場所によっては断層に平行な方向に母岩の 10 倍の大きさの

透水係数が認められた。一方、断層に直交する方向では母岩より 2 オーダー低い透水係

数となった。ずれの大きな断層ほど直交する方向で透水係数が低いことが分かった。代

表的な断層域では、断層に平行な方向の透水係数は亀裂の開口幅に大きく影響されるが

直交方向では全く影響が無い。Jourde らは彼らの提唱する手法は詳細な構造と透水係数

データが入手、あるいは推定可能なあらゆるタイプの断層にも有効であるとしている。

Figure 3-33 詳細な断層ゾーンのマッピングと実データにキャリブレートされたイメージデー

タに基づく透水係数を使った等価な数値モデル(Jourde et al., 2002)

Burbey(2007)はネバダ州の Mesquite で厚い堆積層に新たに掘削された公共井戸を使っ

た 62 日間の帯水層試験中に GPS ネットワークを使って水平方向と垂直方向の地表面変

位のモニタリングデータを採取した。当初は放射方向と接線方向の変位から非等方性の

帯水層だと結論されたが、新たな InSAR（Interferometric Synthetic Aperture Radar）デー

タによる地盤沈下の広がりを解析した結果、当該サイトは傾いた正断層によって北西側

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に境界があり、南側の沈下も井戸の中心からずれていると考えられる。有限要素法を使

い観測された水平方向の変位に合わせるように断層の位置と水圧と応力特性パラメータ

を推定した。数値シミュレーションの結果、井戸と他の GPS ステーションで観測され

た変位の大きさと方向が推定される断層から遠ざかる南東方向である場合、断層の幅は

5m で高い透水性と貯留係数を持ち、北東から南西に伸びていると推測される。変位の

シミュレーション結果は観測データと良く一致することから、断層の存在の推測が正し

い事を示している。帯水層の揚水試験中に水平方向の変位データをモニタリングするこ

とで断層の存在や境界条件について貴重な情報が得られることが示された。

3.2 我が国の事例

3.2.1 超深地層研究所近辺

岐阜県瑞浪市に JAEA（日本原子力研究開発機構）によって瑞浪深地層研究所が建設

されつつある。その主立孔付近には NNW 断層と呼ばれる断層が存在し、掘削や揚水試

験に対する水頭の応答データや地震の際の水位変化、傾斜計データ等の様々な調査デー

タを元に、Takeuchi ら(2007a)はシーケンシャル水理試験（竹内ら、2007b）及び、高精

度傾斜計（中谷ら、2007）を使い NNW 断層は断層を横切る方向に低透水性であると推

定した。

Figure 3-34 NNW 断層を挟んだ両側の地下水頭応答挙動の違い(Takeuchi et al. 2007)

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また、深地層研究所の北には月吉断層が存在する。月吉断層は東西に伸びる長さ約

10km 程度とされる逆断層である。月吉断層が横切る JAEA の正馬様サイトでは前身の

JNC によって月吉断層の水理性状の調査がなされた。その結果、上盤と下盤では約 40m

の水頭差が存在し、断層を横切る方向では低透水性であることが確認された。また、断

層の両側に断層に平行に割れ目帯が発達し、高い透水性を示す事が確認された。Sawada

ら(2005)はこれらの水理地質データを用いて複数の研究機関がそれぞれ独自のモデルを

使った場合のモデル予測の不確実性の検討を行った。また、Doughty and Karasaki (2003)

は月吉断層をコア部と両側のダメージゾーンからなるサンドイッチ構造の断層と仮定し、

9km×9km×2km の 3 次元水理地質モデルを構築し、パッカー除去時の複数のボアホー

ル内の圧力応答分布と温度分布データを使って断層のコア部とダメージゾーンの透水係

数を推定した。

Figure 3-35 Doughty and Karasaki (2003) がモデル化した 9km×9km×2km の東濃地区のモデ

ルの断面。月吉断層はサンドイッチ構造となっている。

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3.2.2 幌延深地層研究サイト

伊藤ら (2005) は幌延深地層研究計画サイト周辺地域の間隙水圧分布データを観測デー

タとして用いた逆解析手法の適用性の検討を行った。その結果、断層の水理地質構造に

よって断層近傍において涵養された地下水の流動状況が大きく異なることが示された。

また、深部の水頭値を観測することによって断層の水理地質構造が推定できる可能性を

示した。

Figure 3-36 低透水ゾーン断層モデルとサンドイッチ構造断層モデルの逆解析水頭値分布の比

較 (伊藤ら、2005)

また、石井ら (2006) は北海道北部、幌延地域の大曲断層の三次元分布とその水理特性

を反射法地震探査と AMT(Audio Frequency Magnetotelluric)法を使って推定した。その結

果、大曲断層は幌延深地層研究センター近辺において地表部で over-step し、地下では収

斂する 3 次元構造を持っていると推定した。また、大曲断層は地下 1km 以浅において高

透水ゾーンとして機能していると推定した。

0 20 40–3000

–2000

–1000

0

Hydraulic Head above surface (m)

Dep

th (m

)

Measured

Calculated with sandwich structure

Lower Quaternary

Yuuchi

Koetoi

Calculated with low permeability fault

97

Figure 3-37 石井ら (2006) が反射法地震探査と AMT 法を使って推定した大曲断層の 3 次元

構造

3.3 ALSM (Airborne Laser Swath Mapping)

ALSM は空中レーザー地形計測を意味し、近年注目を浴びている技術である（Carter

ら、2007）。ALSM は LIDAR (LIght Detection And Ranging あるいは Laser Imaging

Detection And Ranging) とも呼ばれる。１ｍ以下の観測点密度を誇り植生の影響をさほど

受けずに精密な地表面のデジタル標高マップの取得が可能であり、未知の断層の発見や

地すべり危険地域の特定に有用であると考えられる。原理としては航空機搭載のスキャ

ニングレーザーレンジファインダー及びディファレンシャル GPS と慣性ナビゲーショ

ンシステムを組み合わせる事によって、木の枝葉を避けてレーザーの斜め方向からの地

面からの反射波を捉える。Kondo ら（2008）は糸魚川－静岡構造線に沿って LiDAR 測

量を行い、断層の痕跡を確認する事に成功した。また、米国でのデータではあるが、1

平方マイルあたりおよそ$400 から$1,000 の費用で 100 万点のデータが採取可能であり、

コスト的にも非常に有利である。従来の写真測量法に比べて 1 オーダー程度精度が高い。

ただし、植生の影響を受けにくいが、冬の晴れた日に測量するのが望ましい。

98

Figure 3-38 （左）通常の空中写真（中）フィルターを掛けて植生を取り除いた DEM、

（右）断層トレースを重ねた図。http://geomaps.wr.usgs.gov/sfgeo/quaternary/stories/find_faults.html

3.4 断層水理の解析解と数値モデル

3.4.1 断層のある井戸試験の解析解

Anderson (2006)は解析解を用いて断層を横切る方向に透水性の低い断層と透水性の高

い断層のどちらの場合も断層をまたいだ反対側での圧力応答は揚水試験に対して遮水的

な応答を示すと結論付けた。この結果は直感と相反するが、事実である。多くの場合ど

ちらの性状の断層が存在する場合も同様な圧力応答を示す。従って、水頭分布から断層

の性状を定性的、定量的に推定することが一般に行われているが、解析解のような理想

的な条件下でさえ、断層の水理性状の推定が困難であることから、現場試験で限られた

数の観測井の圧力応答から断層が高透水性であるか、低透水性であるかを判断すること

は極めて困難であることが提示された。よって、断層の水理性状を正確に把握する為に

は、断層のコア部とダメージゾーンを直接試験し、観測点を設置する必要がある。無論、

東濃地区の月吉断層に見られるように自然の水頭差があれば、少なくとも直交方向には

低透水性だと推測することが可能である。

99

Figure 3-39 揚水試験に対する(a)低透水性 (b)高透水性 (c) 二面性の断層の反対側の水頭応答

分布。何れも似通った水頭分布を示している。(Anderson, 2006)

3.4.2 不均質な断層のシミュレーション

Lopez と Smith (1996) は断層内部の不均質性がもたらす地下水の複雑な流れと地温勾

配の変化を数値解析を用いて調べた。これらの不均質性は断層の内部と地表面の断層ト

レースの高い透水性の箇所の連続性によってもたらされている。彼らは不均質な断層内

の流れのチャンネリングは均質なケースに比べて最高流出温度に関してはさほどの差は

無いが、地下水の流量が大きくなる為、ローカルな熱流量に関しては大きな差異が出る

ことを示した。

続く第 4 章では、断層内のフラクチャーや小空洞等の不均質性をモデル化して、応力、

熱、二相流の連成的な物理プロセスを具現化する手法を詳しく述べた。

100

Figure 3-40 不均質な断層内の流れのパターン。(a)は母岩の平均透水係数が 10-19m2、(b)は

10-16m2のケース。(Lopez and Smith, 1996)

3.4.3 断層を跨いだ水理試験

セクション 3.1 ~ 3.2 で明らかになったように、断層の水理性状は種々の地質情報や断

層パラメーター、物理探査データからある程度の類推が可能であるが、透水係数分布や

透水係数の絶対値を確認する為には水理試験が欠かせない。ところが、3.4.1 で述べたよ

うに、解析解を用いた研究では断層を跨いだ水理試験では断層の性状を把握するのが困

難であることが示された。本セクションでは 2 次元の数値モデルを使って断層の近傍で

水理試験を行った場合、断層の性状によって水理応答がどのように時系列変化するかを

調べた。

断層の性状については、（１）透水係数が母岩に比べて 2 オーダー低い低透水性断層、

（２）2 オーダー高い高透水性断層、（３）コア部とダメージ部からなるサンドイッチ

構造の断層の 3 つのケースについて調べた。Figure 3-41 にシミュレーションに使用した

2 次元の数値モデルメッシュを示す。Figure 3-42 に 7 日後の圧力分布を示す。Figure 3-43

に各ケースの揚水井と観測井における圧力降下の経時変化を示す。直感的には低透水性

の断層がある場合観測井での圧力変化が少なく、高透水性の場合は、圧力変化が伝わり

易いと考えがちであるが、図から明らかなように、断層の透水性の性状に拘らず、断層

を挟んで揚水井の反対側では圧力が伝わりにくくなっている。

101

Figure 3-41 断層を跨いだ水理試験の数値モデル。4km×4km、10m 格子グリッド。拡大図は

断層の構造を示す。揚水井（矢印）、観測井（虫眼鏡）はそれぞれ断層から

100m の距離。

Figure 3-42 それぞれの断層のケースの揚水に対する圧力分布。断層がある場合は何れの場合

も断層の反対側に圧力が伝わりにくいことがわかる。

高透水サンドイッチ

断層なし低透水

102

.

Figure 3-43 各ケースの圧力降下の時間変化。観測井では何れのケースも断層がないケースに

比べて圧力降下度が低いことがわかる。

3.5 まとめ

本章では断層の性状の調査事例や、調査技術の実例をまとめた。断層の種類や地質に

よって、あるいはその地域によって断層性状の特定に有効である各種の技術が見られた。

特に、我が国に於いて概要調査で求められるスケールでは、物理探査技術においては、

AMT（Acoustic frequency Magneto Telluric）、反射法地震波探査、LIDAR (LIght

Detection And Ranging) が断層の特定に有効であった例が多く見られた。また、断層は地

域の水理場に大きな影響を及ぼしているので、地域の水頭分布が精密に得られれば、断

層の水理性状が推定できることが判った。さらに、地化学的情報や、温度分布から断層

性状が推定出来た事例が幾つか見られた。しかしながら、従来の方法や配置では、揚水

試験を行っても揚水井や観測井の圧力降下の経時変化のみからは断層の存在そのものは

推定できる可能性はあるが、性状の推定は難しい事が明らかになった。

0

20

40

60

80

100

120

0 5 10 15 20 25 30

Time (days)

Draw

dow

n (m

)

No Fault (P)No Fault (E)Low K (P)Low K (E)High K (P)High K (E)SandW (P)SandW (E)

103

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4 Conceptualization and Modeling of Fault Zones

4.1 Introduction

Since the 1960s, a number of numerical approaches and techniques have been

developed and applied for modeling flow and transport processes in fractured reservoirs

(e.g., Kazemi, 1969; Pruess and Narasimhan, 1985; Wu and Pruess, 1988). Even with the

significant progress that has been made towards understanding and modeling of flow and

transport processes in fractured rock so far, most studies have focused primarily on

naturally fractured reservoirs, without explicitly taking faults into consideration. Recently,

characterizing the fractured rock of faults or fault zones has received attention, because

fault zones are found to be closely associated with, and may dominate, flow and transport

processes in fractured reservoirs (Wu et al., 2004; 2006a; 2007a).

Mathematical approaches developed for modeling flow through fractured

reservoirs rely in general on continuum approaches. Such approaches involve developing

conceptual models, incorporating the geometrical information of a given fracture-matrix

system, setting up mass and energy conservation equations for fracture-matrix domains,

and then solving discrete nonlinear algebraic equations of mass and energy conservation.

The commonly used mathematical methods for modeling flow through fractured rock

include: (1) an explicit discrete-fracture and matrix model (Snow, 1965), (2) a dual- and

multiple-continuum method, including double- and multiporosity, dual-permeability, or

the more general “multiple interacting continua'” (MINC) method (Warren and Root,

1963; Kazemi, 1969; Pruess and Narasimhan, 1985; Wu and Pruess, 1988), and (3) an

effective-continuum method (ECM) (Wu, 2000a).

In addition to the traditional double-porosity concept, a number of triple-porosity

or triple-continuum models have been proposed (Closemann, 1975; Wu et al., 2004a;

Kang et al., 2006) to describe flow through fractured rocks. In particular, Liu et al. (2003)

and Camacho-Velazquez et al. (2005) present several new triple-continuum models for

single-phase flow in a fracture-matrix system that include cavities within the rock matrix

(as an additional porous portion of the matrix). In general, these models have focused on

handling the heterogeneity of the rock matrix or fractures, e.g., subdividing the rock

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matrix or fractures into two or more subdomains with different properties. In concept, all

these approaches can be applied to modeling flow and transport in highly dense fractures

in fault zones.

Dual-continuum or multiple-continuum approaches for modeling flow in fractured

porous media, as applied in this study, include the classical double-porosity model

(Barenblatt et al., 1960; Warren and Root, 1963), the dual-permeability concept, the more

rigorous dual-continuum generalization of the MINC (Pruess and Narasimhan, 1985) and

the multicontinuum model (Wu and Pruess, 1988). In the double-porosity model, a flow

domain is composed of matrix blocks with low permeability, embedded in a network of

interconnected fractures. Global flow and transport in the formation occur only through

the fracture system, conceptualized as an effective continuum. This model treats matrix

blocks as spatially distributed sinks or sources to the fracture system without accounting

for global matrix-matrix flow. In comparison, the MINC concept is able to describe

gradients of pressures, temperatures, or concentrations near the matrix surface and inside

the matrix—by further subdividing individual matrix blocks with one- or

multidimensional strings of nested meshes. Therefore, the MINC model in general

provides a better numerical approximation for transient fracture-matrix interactions than

the double-porosity model. Because of its computational efficiency and its ability to

match many types of observed fault-flow data (e.g., Wu et al., 2004; 2007a), the dual-

continuum models as exemplified in the double-porosity and dual-permeability

concepts—which have perhaps been the most widely used methods in petroleum and

geothermal engineering, as well as in groundwater hydrogeology—will be used for this

study.

In this section, a physically based fault conceptual model is presented for

modeling multiphase flow and transport processes in the fractured rock of fault zones. In

particular, we discuss a general mathematical framework model for dealing with fracture-

matrix interactions, which is applicable to both continuum and discrete fracture

conceptualization in fault zones. The multicontinuum, physically based conceptual

mathematical model includes the effects of various scaled fractures, vugs, or cavities on

water and heat flow processes in fault zones.

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In this continuum model, faults or fault zones of formations is conceptualized as a

multiple-continuum medium, consisting of (1) highly permeable, large-scale and well-

connected fractures, (2) low-permeability rock matrix, and (3) various-sized vugs (if

existing). Similar to the conventional double-porosity model, the large-scale, well-

connected fracture continuum is responsible for global flow within faults, while vuggy

and matrix continua, providing storage space, are locally connected to each other (and

interacting with globally connecting fractures). In addition, the flow along faults is also

interacting laterally with formation layers on both sides of the fault zone.

In this approach, a subsurface fault domain consisting of fractures, rock matrix, vugs, or

other large pores is discretized using an unstructured grid with regular or irregular

meshes, followed by time discretization carried out using a backward, first-order, finite-

difference method. The final discrete nonlinear equations are handled fully implicitly,

using Newton iteration. In addition, the fracture medium is handled using a general dual-

continuum concept with continuum or discrete modeling approaches.

We demonstrate that with this unified approach, modeling a particular process of fracture

or porous-medium flow and transport in fault zones becomes simply a matter of defining

types of media (i.e., fractures, matrix, vugs, or other large pores) with a set of state

variables, along with their interactions on the interfaces between continua. The proposed

numerical scheme is applicable to simulating water and heat flow, as well as solute

transport, through fractured fault zones and their interaction with surrounding rock layers.

4.2 Conceptual Model of Fractured Faults

Faults may consist of a single fracture or multiple scale fractures, or highly dense

fractured zones. In general, faults or fault zones belong typically to fractured rock and

can be classified as a special case of fractured reservoirs. A typical fractured reservoir, as

shown in Error! Reference source not found. (Warren and Root, 1963), consists of

large-scale fractures, a low-permeable rock matrix, and a number of various sized cavities

or vugs. Figure 4-2 shows a formation of outcrops with a vertical fault (Wu et al., 2006),

and the fractured system is conceptualized using vertical and horizontal fracture networks

with vugs along the vertical fault in the figure.

112

Figure 4-1 Schematic of conceptualizing fractured formation using the double-porosity conceptual model (Warren and Root, 1963).

vugs matrix matrixfracture

fracture

113

Figure 4-2 Schematic of conceptualizing vuggy fractured formation as a discrete fracture system with well connected, (a) outcrop pictures and (b) conceptual model (Wu et al. 2006).

114

As shown in

Figure 4-1 and Figure 4-2, different types of fractured rock in fault zones can be described

using a multicontinuum concept. The multicontinuum conceptual model considers large

fractures as main pathways for the global flow. Vuggy and matrix continua, locally

connected to each other as well as directly or indirectly interacting with globally

connecting fractures, provide storage space as sinks or sources to fractures. Note that

vugs directly connected with fractures could be considered part of the fracture continuum.

More specifically, we conceptualize the fractured-vug-matrix system of fault zones as

consisting of (1) a fracture continuum—“large” fractures (or fractures), globally

connected on the scale of a model domain, providing flow paths to injection and

production wells; (2) a vuggy continuum—various-sized vugs or large pore space cavities,

which are locally connected to fractures either through “small” fractures or isolated by

rock matrix; (3) a matrix continuum—rock matrix, which may contain a number of

cavities, locally connected to large fractures and/or to vugs; and (4), small-scale fractures

(Wu et al., 2004a).

In principle, the proposed multiple-continuum model for fault zones is a natural

extension of the generalized multiple-continuum (MINC) approach (Pruess and

Narasimhan, 1985; Wu and Pruess, 1988). In this approach, an “effective” porous

medium is used to approximate fractures, vugs (if any), rock matrix continuum, or any

large pores. The triple- or multiple-continuum conceptual model assumes that

approximate thermodynamic equilibrium exists locally within each of the continua at all

times. Based on this local equilibrium assumption, we can define thermodynamic

variables, such as pressure, fluid saturation, concentration, and temperature, for each

continuum. Note that the multiple-continuum model is not limited to the orthogonal

idealization of the fracture system, or uniform size, regular shape, or distribution of vugs

and cavities, as illustrated in

Figure 4-1 and Figure 4-2. Irregular and stochastic distributions of fractures and cavities

can be handled numerically, as long as the actual distribution patterns are known (Pruess,

1983).

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4.3 Mathematical Model

The physical processes associated with flow and transport in fractured porous media in

fault zones are governed by the same fundamental conservation laws as those used in

other branches of science and engineering: conservation of mass, momentum, and energy

governs the behavior of fluid flow, chemical transport, and heat transfer in porous or

fractured rock. These physical laws are often represented mathematically on the

macroscopic level by a set of partial differential or integral equations, called governing

equations. These governing equations are generally nonlinear as long as compressible

fluid flow or heat transfer is involved and needed to quantitatively model the flow and

transport processes occurring in porous or fractured media. Based on the general

conservation laws, we present a set of generalized governing equations for fluid flow,

multicomponent transport, and heat transfer in porous and fractured media, providing a

framework for numerical formulations to cover all possible scenarios for flow and

transport in porous media.

We consider the following physical processes in fault zones: (1) single-phase

aqueous phase flow, (2) solute (multiple minerals) transport, and (3) ambient heat flow,

driven by geothermal gradient. Let us consider a nonisothermal system consisting of one

aqueous fluid phase, which in turn consists of a number of mass components. To derive a

set of generalized governing equations for fluid flow, multicomponent transport, and heat

transfer, we assume that these processes can be described using a continuum approach

within a representative elementary volume (REV) in a porous or fractured medium. In

addition, a condition of local thermodynamic equilibrium is assumed, so that at any time

temperatures, phase pressures, densities, viscosities, enthalpies, internal energies, and

component concentrations (or mass fractions) are the same locally in each REV of the

porous medium.

According to mass and energy conservation principles, we can write a generalized

conservation equation of mass components and energy in a porous continuum, as follows:

kkkk

FqGt

M++=

∂∂ (1)

116

where superscript k is the index for the components k = 1, 2, 3,…, Nc, with Nc being the

total number of mass components and k = Nc+1 for an energy “component” (note that

heat energy is here regarded as a component for convenience); M is the accumulation

term of component k; kG is the decay or internal generation (reaction) term of mass or

energy components; kq is an external source/sink term or fracture-matrix exchange term

for mass or energy component k and energy; and kF is the “flow” term of mass or energy

movement or net exchange from single-phase flow, or diffusive and dispersive mass

transport, or heat transfer, as discussed below.

In addition to the conservation or continuity equations of mass and thermal energy,

shown in Equation (1), we also need specific relationships or mechanisms that describe

why and how fluid flow, solute transport, and heat transfer occur in porous and fractured

media. This is to define the “flow” term in Equation (1), and the following specific laws

act as such mechanisms by governing local fluid flow, component transport, and heat

transfer processes in porous media.

4.3.1 SINGLE-PHASE DARCY FLOW

For single-phase liquid flow, the accumulation terms in Equation (1) for water

phase is evaluated as

φρ= wwM (2)

where wρ is the density of water phase; and φ is the porosity of porous or

fractured media. Note that in this special case, the decay or generation term is negligible

with

0G w = (3)

The mass flow term is determined by

( )vwwF vρ∇= • (4)

where wv is a vector of the Darcy’s velocity or volumetric flow of water, defined

by Darcy’s law to describe the flow of single fluid as

( )zgPkwww ∇ρ−∇

μ−=v (5)

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where wP , μ , and g are pressure, viscosity of water phase, and gravitational constant,

respectively; z is the vertical coordinate; k is absolute or intrinsic permeability (a tensor

in general).

4.3.2 MASS TRANSPORT

The movement of dissolved mass components or chemical species in a fluid-rock-

medium system can also be handled as a special case of Equation (1). The accumulation

terms for component k is

( ) kd

kwws

kww

k KX1XM ρρφ−+φρ= (k = 1, 2, 3,…, Nc) (6)

where κwX is the mass fraction of component k in water, sρ is the density of rock solids;

and kdK is the distribution coefficient of component k between the aqueous phase and

rock solids to account for adsorption effects.

In the case in which components are subject to a first-order radioactive decay, the

decay/generation term is

( )( )kd

kwws

kwwk

k KX1XG ρρφ−+ρφλ= (k = 1, 2, 3,…, Nc) (7)

where kλ is the radioactive decay constant of component k.

The mass component transport is governed in general by processes of advection,

diffusion, and dispersion, and is also subject to other processes such as radioactive decay,

adsorption, dissolution and precipitation, mass exchange and partition between phases, or

chemical reactions. Advective transport of a component or solute is carried by flow of a

fluid, and diffusive and dispersive flux is contributed by molecular diffusion and

mechanical dispersion, or hydrodynamic dispersion. These processes are described using

a modified Fick’s law for transport through a single-phase porous medium (Wu, 2004b).

Then, the total mass flow term for a component k, by advection and dispersion, is written

as

( ) ( )kww

kww

kww

k XDXF ρ∇+ρ•−∇= •v (k = 1, 2, 3,…, Nc) (8)

Equation (8) indicates that the mass flow consists of two parts: the first part, i.e.,

the first term on the right-hand side of (8), is contributed by advection in water flow; and

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the second part [the second term on the right-hand side of (8)] is the diffusive flux by

hydrodynamic dispersion. In Equation (8), kwD is the hydrodynamic dispersion tensor

accounting for both molecular diffusion and mechanical dispersion for component k in

the water phase, defined by an extended dispersion model (Scheidegger, 1961),

( ) jikw

w

wwTLjiwT

kw dτD δφ+α−α+δα=

vvvv (k = 1, 2, 3,…, Nc) (9)

where Tα and Lα are transverse and longitudinal dispersivities, respectively, in the water

phase of porous or fractured media; τ is tortuosity of the porous medium; kwd is the

molecular diffusion coefficient of component k within the water phase; and δij is the

Kronecker delta function (δij = 1 for i = j, and δij = 0 for i ≠ j), with i and j being

coordinate indices.

4.3.3 HEAT TRANSFER

The accumulation term for the heat equation is usually is defined as

( ) ssww1N U1UM c ρφ−+ρφ=+ (10)

where sUandUβ are the internal energies of water phase and rock solids, respectively.

Heat transfer in porous and fractured media is in general a result of both

convective and conductive processes. Heat convection is contributed by thermal energy

carried mainly by bulk flow of water. On the other hand, heat conduction is driven by

temperature gradients and may follow Fourier’s law. Then the combined, overall heat-

flux term, owing to convection, conduction, and radiation in a multiphase,

multicomponent, porous medium system, may be described as:

( ) ( )TKhF T1Nc ∇∇+ρ•∇−= •βββ+ v (11)

where wh are the specific enthalpies of the water phase, TK is the overall thermal

conductivity; and T is temperature;

119

4.3.4 CONSTITUTIVE RELATIONSHIPS

To complete the mathematical description of water flow, multicomponent

transport, and heat transfer in porous and fractured media, Equation (1), a generalized

mass- and energy-balance equation, needs to be supplemented with a number of

constitutive equations. These constitutive correlations express the interrelationships of,

and constraints on, physical processes, variables, and parameters, and allow the

evaluation of secondary variables and parameters as functions of a set of primary

unknowns or variables selected to make the governing equations solvable. Many of these

correlations for estimating properties and interrelationships are determined by

experimental studies.

4.4 Numerical Formulation and Solution

The methodology for using numerical approaches to simulate subsurface flow and

transport, and heat transfer, consists in general of the following three steps: (1) spatial

discretization of mass and energy conservation equations, (2) time discretization; and (3)

iterative approaches to solve the resulting nonlinear, discrete algebraic equations. Among

various numerical techniques for simulation studies, a mass- and energy-conserving

discretization scheme, based on finite or integral finite-difference or finite-element

methods, is the most commonly used approach and is discussed here.

4.4.1 DISCRETE EQUATIONS

The component mass- and energy-balances represented in Equation (1) are

discretized in space using a control-volume concept. The control-volume concept

provides a general spatial discretization scheme that can represent a one-, two- or three-

dimensional domain using a set of discrete meshes. Each mesh has a certain control

volume for a proper averaging or interpolation of flow and transport properties or

thermodynamic variables. The control volume concept includes the conventional finite-

difference scheme (Aziz and Settari, 1979; Narasimhan and Witherspoon, 1976; Pruess et

al., 1999), an integral finite-difference method (Figure 4-3), a control-volume finite

element (Forsyth, 1994), and Galerkin finite-element methods (Huyakorn et al., 1994).

120

These are the most widely used discretization schemes for subsurface flow and transport

simulation.

Figure 4-3 Space discretization and flow-term evaluation in the integral finite difference method (Pruess et al., 1999)

As shown in Figure 4-3, the spatial discretization of Equation (1) is carried out

using the integrated finite difference scheme, and time discretization is carried out using a

backward, first-order, fully implicit finite-difference scheme. The discrete nonlinear

equations for components of water, other mass components, and heat at gridblock or node

i can be written in a general form:

{ } 1n,ki

j

1n,kij

in,ki

1n,ki

1n,ki Qflow

tVMtGM

i

+

η∈

+++ +=Δ

−Δ+ ∑ (12)

(k = 1, 2, 3, …, Nc, Nc+1) and (i=1, 2, 3, …, N)

where superscript k serves also as an equation index for all mass components with k = 1,

2, 3, …, Nc and k = Nc+1 denoting the heat equation; superscript n denotes the previous

time level, with n+1 the current time level to be solved; subscript i refers to the index of

gridblock or node i, with N being the total number of nodes in the grid; Δt is the time step

size; Vi is the volume of node i; ηi contains the set of direct neighboring nodes (j) of node

i; and kiA , k

iG , kijflow , and k

iQ are the accumulation and decay/generation terms,

respectively, at node i.

121

The “flow” term between nodes i and j, and the sink/source term at node i for component

k or thermal energy, respectively, are defined below. Equation (12) has the same form

regardless of the dimensionality of the system, i.e., it applies to one-, two-, or three-

dimensional flow, transport, and heat-transfer analyses within and outside fault zones.

The accumulation and decay/generation terms for mass components or thermal

energy are evaluated using Equations (6), (7), and (10), respectively, at each node i. The

“flow” terms in Equation (12) are generic and include mass fluxes by advective and

dispersive processes, as described by Equation (4) or (8), as well as heat transfer,

described by (11).

The mass flow term of Equation (12) for single-phase water flow is described by a

discrete version of Darcy’s law, i.e., the mass flux of water phase along the connection is

given by

[ ]ijji2/1ij,wwijflow ψ−ψγλ= + (13)

where λw,i j+1/2 is the mobility term to water phase, defined as

2/1ij

w2/1ji,w

++ ⎟⎟

⎠

⎞⎜⎜⎝

⎛μρ

=λ (14)

In Equation (13), ijγ is transmissivity and is defined differently for finite-

difference or finite-element discretization. If the integral finite-difference scheme (Pruess

et al., 1999) is used, the transmissivity is evaluated as

ji

2/1jijiji DD

kA+

=γ + (15)

where ijA is the common interface area between connected blocks or nodes i and j

(Figure 4-3); and Di is the distance from the center of block i to the interface between

blocks i and j (Figure 4-3). The flow potential term in Equation (13) is defined as

i2/1ji,wiwi ZgP +ρ−=ψ (16)

where Zi is the depth to the center of block i from a reference datum.

122

For mass-component transport, the flow term, or the net mass flux by advection

and hydrodynamic dispersion of a component along the connection of nodes i and j, is

determined by k

ijD,k

ijA,kij FFlowf += (k = 1, 2 , 3, …, Nc) (17)

where kij,AF and k

ij,DF are the net mass fluxes by advection and hydrodynamic dispersion

along the connection, respectively, with

( ) wji2/1ij

kij

kij,A flowXAF

+β= (18)

and

( )kww

kwijij

kij,D XDAF ρ∇−= ••n (19)

where nij is the unit vector along the connection of the two blocks i and j.

The total heat flux along the connection of nodes i and j, including advective and

conductive terms, can be also evaluated, when using a finite-difference scheme, by

( ) ( )⎥⎥⎦

⎤

⎢⎢⎣

⎡

+−

+= +++

ji

ij2/1ijTij

wji2/1ijw

1Nij DD

TTKAflowhlowf c (20)

In evaluating the “flow” terms in the above Equations (13)–(16), (18), and (20),

subscript ij+1/2 is used to denote a proper averaging or weighting of fluid flow,

component transport, or heat-transfer properties at the interface or along the connection

between two blocks or nodes i and j. The convention for the signs of flow terms is that

flow from node j into node i is defined as “+” (positive) in calculating the flow terms. Wu

and Pruess (2000b) present a general approach to calculating these flow terms associated

with advective and dispersive mass transport and heat transfer in a multiphase system,

using an irregular and unstructured, multidimensional grid.

The mass or energy sink/source in Equation (12) at node i, kiQ , is defined as the

mass or energy exchange rate per unit volume of rock or soils. It is normally used to treat

boundary conditions, such as surface infiltration, pumping, and injection through wells.

Note that we present explicit, discrete expressions for estimating all the flow terms above,

except for dispersive fluxes in Equation (18). This is because of the numerical difficulties

123

introduced in handling the hydrodynamic tensor of dispersion, which is treated very

differently with different numerical approaches, such as finite difference or finite element.

In most formulations for solute transport, the off-diagonal terms and contributions of the

dispersion tensor are ignored, and dispersive transport is considered only along the

principal directions. However, a general procedure for using the integral finite difference

to incorporate a full dispersion tensor is presented by Wu and Pruess (2000b).

Note that Equation (12) presents a precise form of the balance equation for each

mass component and heat in a discrete form. It states that the rate of change in mass or

energy accumulation (plus decay/generation, if existing) at a node over a time step is

exactly balanced by inflow/outflow of mass and energy, and also by sink/source terms,

when existing for the node. As long as all flow terms have flow from node i to node j

equal to and opposite to that of node j to node i for fluids, components, and heat, no mass

or energy will be lost or created in the formulation during the solution. Therefore, the

discretization in (12) is conservative.

4.4.2 NON-DARCY AND OTHER COMPLEX FLOW

The flow regime may be more complicated within faults or fault zones, because of

(1) the high permeability of fractures and (2) large pores, such as vugs and larger-

aperture fractures, in fault zones.

Non-Darcy Flow: In addition to Darcy flow, as described in Equations (5) or (13), non-

Darcy flow may also occur between and among the multiple continua within fault zones.

A general numerical approach for modeling non-Darcy flow (Wu, 2002) can be directly

extended to the multiple-continuum model of this work for flow in fault zones.

Volumetric flow rate (namely Darcy velocity for Darcy flow) for non-Darcy flow of each

fluid may be described using the multiphase extension of the Forchheimer equation:

( ) wwwwww kP vvvg βρ+

μ=ρ−∇− (21)

β is the non-Darcy flow coefficient, intrinsic rock property, with a unit m-1 for water phase

under flow condition.

124

Note that for no matter what type of flow, (i.e., Darcy flow, non-Darcy flow, or

the following pipe-type flow), the discrete mass- and energy-balance equation of (12) is

always valid. For the case of non-Darcy’s flow, the flow term ( wjiflow ) in Equation (13)

along the connection (i, j), between elements i and j, is numerically replaced by (Wu,

2002):

( ) ( )⎪⎭

⎪⎬

⎫

⎪⎩

⎪⎨

⎧

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡ψ−ψγ−⎟

⎟⎠

⎞⎜⎜⎝

⎛

λ+

λ−

β= ββ

ββ+

2/1

ijij

2

2/1ij

ijwji

11k2A

flow (22)

( )ji

2/1ijw2

ji DDk4

+

βρ=γ + (23)

Flow in Parallel-Wall Fracture or Tube: In general, flow along connecting paths of

large-aperture fractures or vugs through narrow pores or fractures may be too fast, or

openings may be too large to describe using Darcy’s law. In particular, when these large-

aperture fractures vuggy connections could be approximated as a single (or parallel)

fracture or tube within fault zones, solutions of flow through a parallel-wall, uniform

fracture or Hagen-Poiseuille tube-flow solution (Bird et al., 1960) may be extended to

describe such flow in Equation (13):

( )ji

3

ji DD12wb+

=γ for fracture-type connection (24)

and

( )ji

4

ji DD8r+

π=γ for tube-type connection (25)

where b is fracture aperture, w is fracture width, and r is tube radius. Similarly, flow

solutions for both laminar and turbulent flow through simple geometry of vug-vug

connections can be used for flow between these vuggy connections.

4.4.3 EFFECT OF ROCK DEFORMATION

Based on the observation from experimental results (Wu et al., 2008) and

previous research (Terzaghi, 1943), the effective porosity and permeability of porous and

125

fractured rock in an isothermal system are assumed to correlate with the mean effective

stress (σ’m), defined as:

wwmm P)P,z,y,x(' α−σ=σ (26)

where σm is mean total stress; and α is Biot’s effective parameter (Biot, 1941) and is

treated as constant in space or in general estimated by

3/))P,z,y,x()P,z,y,x()P,z,y,x(()P,z,y,x( wzyxwm σ+σ+σ=σ (27)

where σx, σy, and σz are total stress in x, y, and z- directions, respectively.

With the definition of the mean effective stress of Equation (26), the effective

porosity of fractures and rock matrix in fault zones is defined as a function of mean

effective stress only,

( )m'σφ=φ (28)

Similarly, intrinsic permeability is related to the effective stress as,

( )m'kk σ= (29)

To incorporate rock-deformation effects at a given site, the proposed models,

derived from Equations (28) and (29), need to be determined from laboratory or field

studies. The key for applicability of these models, in fault flow simulations coupled with

rock deformation, is that the distribution of effective stress or total stress field must be

predetermined as a function of spatial coordinates and pressure fields, as in Equations

(26) and (27). In practice, the stress distribution may be estimated analytically,

numerically, or from field measurements, because in reservoirs, changes in effective

stress are primarily caused by changes in pressure.

When the functions of (28) and (29) are determined, then the two equations are

used in flow and transport calculations. For example, Equation (28) is used in (6) and

(10) for accumulation terms, while (29) is used in (13) or (22) for flow calculation to

incorporate effect of rock deformation of fault flow.

126

4.4.4 HANDLING FRACTURE-VUG-MATRIX INTERACTION IN FAULTS

The technique used in this work for handling flow through vuggy fractured rock

follows the dual- or multiple-continuum methodology (Warren and Root, 1963; Pruess

and Narasimhan, 1985; Wu and Pruess, 1988). With this dual-continuum concept,

equations for fluid and heat flow and mass transport discussed above can be used to

describe flow along fractures and inside matrix blocks, as well as fracture-matrix-vug

interaction. However, special attention needs to be paid to interporosity flow in the

fracture-matrix-vug continua. Flow terms of interporosity between fracture-matrix,

fracture-vug, vug-vug, and vug-matrix connections are all evaluated using Equation (13)

or (22). However, the transmissivity of (15) will be evaluated differently for different

types of interporosity flow. For fracture-matrix Darcy flow, ijγ , is given by (Wu et al.,

2006b).

FM

MFMFM

kAl

=γ (30)

where FMA is the total interfacial area between fractures (F) and the matrix (M) elements;

kM is the absolute matrix permeability; and FMl is the characteristic distance for flow

crossing fracture-matrix interfaces. For fracture-vug flow, ijγ is defined as

FV

VFV

FV

kA

l=γ (31)

where FVA is the total interfacial area between the fracture and vugs (V) elements; FVl is

a characteristic distance for flow between fractures and vugs; and kV is the absolute

vuggy permeability, which should be the permeability of small fractures that control flow

between vugs and fractures. Note that for the domain in which vugs are isolated from

fractures, no fracture-vug flow terms need to be calculated, because they are indirectly

connected through the matrix.

For vug-matrix flow, ijγ is evaluated as:

127

VM

MVM

VM

kA

l=γ (32)

where VMA is the total interfacial area between the vug and matrix elements; and VMl is

a characteristic distance for flow crossing vug-matrix interfaces. Similarly, the

transmissivity between vugs, when they are connected through narrow fractures or tube

can be defined.

Note that Table 4-1 summarizes several simple models for estimating

characteristic distances in calculating interporosity flow within fractures, vugs, and the

matrix. In such cases, we have regular one-, two-, or three-dimensional large fracture

networks, each with uniformly distributed small fractures connecting vugs or isolating

vugs from fractures, based on the quasi-steady-state flow assumption of Warren and Root

(1963).

In practical application of the proposed modeling approach, the MINC concept

(Pruess, 1983; Pruess and Narasimhan, 1985) is extended to modeling flow through

fractured-vuggy rock. Using this approach, we start with a primary or single-continuum

medium mesh that uses bulk volume of formation and layering data. Then, geometric

information for the corresponding fractures and vugs within each formation subdomain of

fault zones and their surrounding rock is used to generate integrated finite-difference

meshes from the primary grid. Fractures are lumped together into the fracture continuum,

while vugs with or without small fractures are lumped together into the vuggy continuum.

The rest is treated as the matrix continuum. Connection distances and interface areas are

then calculated accordingly (e.g., using the relations discussed above and the geometric

data of fractures and vugs). Once a proper mesh for a multiple-continuum system is

generated, fracture, vuggy, and matrix blocks are specified, separately, to represent

fracture or matrix continua.

128

Table 4-1. Characteristic distances* for evaluating flow terms between fractures, vugs, and matrix systems

Fracture Sets

Dimensions of Matrix

Blocks (m)

Characteristic F-M Distances

(m)

CharacteristicF-V Distances

(m)

Characteristic V-M Distances 1

(m)

Characteristic V-M Distances 2

(m)

1-D A 6/AFM =l xFV ll = 6/aVM =l ( ) 2/dA cVM −=l

2-D A, B )BA(4/

ABFM

+=l

2

yxVF

lll

+= )ba(4/

abVM

+=l

4

d2BA cVM

−+=l

3-D A, B, C )CABCAB/(

10/ABC3FM

++=l

3zyx

fF

llll

++=

)cabcab/(10/abc3VM

++=l

6d3C-BA c

fF++

=l

* Note in Table 4-1, A, B, and C are dimensions of matrix blocks along x, y, and z directions, respectively. 1 Characteristic V-M distances are estimated for the case that vuggy-matrix connections are dominated by small fractures, where dimensions a, b, and c are fracture-spacings of small fractures along x, y, and z directions, respectively. 2 Characteristic V-M distances are used for the case that vugs are isolated from fractures. In addition to discretization techniques discussed above, the following assumption may be also used: there is equilibrium within vugs, i, e., no flow calculations are needed within vugs.

4.5 Numerical Solution Scheme

A number of numerical solution techniques have been developed in the literature

over the past few decades to solve the nonlinear, discrete equations of reservoir

simulations. When handling coupled flow, transport, and heat transfer in a subsurface

system, the predominant approach is to use a fully implicit scheme. This scheme is best

because of the extremely high nonlinearity inherent in those discrete equations, and the

many numerical schemes with different levels of explicitness that fail to converge in

practice. In this section, we discuss a general procedure to solve the discrete nonlinear

Equation (12) fully implicitly, using a Newton iteration method.

Let us write the discrete nonlinear Equation (12) in a residual form as:

{ } 0Qflowt

VMGMR 1n,ki

j

1n,kij

in,ki

1n,ki

1n,ki

1n,ki

i

=−−Δ

−+= +

η∈

++++ ∑ (33)

k = 1, 2, 3, …, Nc +1; i = 1, 2, 3, …, N).

Equation (33) defines a set of (Nc+1) × N coupled nonlinear equations that need

to be solved for every balance equation of mass components and heat, respectively. In

129

general, (Nc+1) primary variables per node are needed to use the Newton iteration for the

associated (Nc+1) equations per node. The primary variables are usually selected among

fluid pressures, mass (mole) fractions of components in fluids, and temperatures. The rest

of the dependent variables—such as viscosity and densities, partitioning coefficients,

specific enthalpies, thermal conductivities, dispersion tensor, and nonselected pressures

and mass (mole) fractions—are treated as secondary variables, which are calculated from

selected primary variables.

In terms of the primary variables, the residual equation, Equation (22), at a node i

is regarded as a function of the primary variables at not only node i, but also at all its

direct neighboring nodes j. The Newton iteration scheme gives rise to

( )( ) ( )p,m1n,k

i1p,mm m

p,m1n,k

i xRxx

xR ++

+

−=δ∂

∂∑ (34)

where xm is the primary variable m with m = 1, 2, 3, …, Nc+1, respectively, at node i and

all its direct neighbors; p is the iteration level; and i =1, 2, 3, …, N. The primary variables

in Equation (34) need to be updated after each iteration:

1p,mp,m1p,m xxx ++ δ+= (35)

The Newton iteration process continues until the residuals 1n,knR + or changes in

the primary variables 1p,mx +δ over an iteration are reduced below preset convergence

tolerances.

Numerical methods are generally used to construct the Jacobian matrix for

Equation (34), as outlined in Forsyth et al. (1995). At each Newton iteration, Equation

(34) represents a system of (Nc+1) × N linearized algebraic equations with sparse

matrices, which are solved by a linear equation solver.

4.6 Treatment of Initial and Boundary Conditions

A set of initial conditions is required to start a transient simulation, i.e., a

complete set of primary variables need to be specified for every gridblock or node. A

commonly used procedure for specifying initial conditions is the restart option, in which

130

a complete set of initial conditions or primary unknowns is generated in a previous

simulation, with proper boundary conditions described.

When using a block-centered grid, first-type or Dirichlet boundary conditions can

be effectively treated with the “inactive cell” or “big-volume” method, as normally used

in the TOUGH2 code (Pruess et al., 1999). In this method, a constant

pressure/concentration/temperature node is specified with an inactive cell or with a huge

volume, while keeping all the other geometric properties of the mesh unchanged.

With finite-element or edge-centered finite-difference grids, first-type boundary

conditions and Neuman boundary conditions can be treated using a generalized,

sink/source term approach (Wu et al., 1996). Certain flux-type boundary conditions are

easy to handle for a situation in which flux distribution along the boundary is known,

such as in dealing with surface infiltration. However, a description of more general types

of flux or mixed boundaries, such as seepage faces and multilayered wells, is part of the

solution, and general procedures for handling such boundary conditions are discussed in

Wu et al. (1996).

4.7 Simulation Example

In this section, we demonstrate the application of the mathematic model discussed

above. The reservoir simulator used is the modified TOUGH2 code-EOS3 module,

incorporating density-dependent variation in the water pressure or head. In modeling, we

use the modified TOUGH2-EOS3 to simulate nonisothermal flow of single-phase water

with density dependence on mineral compositions, in addition to pressure and

temperature in the two wells, HDB-7 and HDB-8. The two wells are located close to each

other, but are possibly separated by a fault (Figure 4-4).

131

Figure 4-4 HDB boreholes at the Horonobe URL site (Yabuuchi et al., 2006)

The basic assumption in the modeling exercise is that the system is in steady-state

condition for water flow, solute transport, and heat flow. In particular, the aqueous

mineral-concentration distribution is assumed at steady state as a function of elevation or

depth only for each well. Therefore, the water density is correlated to mineral

compositions by extrapolating and interpolating the measured mineral compositional data

from the two wells, in addition to its dependence on pressure and temperature.

The two wells are represented using two 1-D column grids of 600 m with uniform

column grid of 5 m grid spacing, starting from zero depth at ground surface, using a

single-continuum, multilayered modeling approach. The geologic units/layers associated

with the two 1-D models are shown in Table 4-2 and Error! Reference source not

found., respectively, for Well HDB-7 and HDB-8. For Well HDB-7, the 1-D model grid

starts from zero depth at ground surface (elevation of 43.752 m) down to 600 m in

depth—the geologic unit (Table 4-2) associated from 0 to 400 m is the Yuuch F, and

from 400 m down, it is the Koetoi unit. For Well HDB-8, the 1-D model covers from the

surface, at elevation of 70.051 m, down to a 600 m depth, with the top layer of 140 m

132

belonging to the Koetoi unit and the unit below belonging to the Wakkanai F. (Error!

Reference source not found.).

Table 4-2. Geologic units/layers and measured temperature and pressure data for Well HDB-7

Geologic Units/Layers

Depth (m)

Thickness (m) Temperature (oC)

Pressure (KPa)

Yuuchi F. 45.35 97.71 8.55 549.27 Yuuchi F. 143.06 48.74 12.2 1600.14 Yuuchi F. 191.80 15.26 14.11 2099.64 Yuuchi F. 207.06 15.04 14.62 2259.53 Yuuchi F. 222.09 10.76 15.13 2414.1 Yuuchi F. 232.86 98.02 15.55 2529.13 Yuuchi F. 330.88 25.77 19.53 3522.07 Yuuchi F. /Koetoi F.

356.64 91.53 20.56 3790.36

Koetoi F. 448.17 75.33 24.51 4709.45

133

Table 4-3 Geologic units/layers and measured temperature and pressure data for Well HDB-8

Geologic

Units/Layers

Depth (m)

Thickness (m) Temperature (oC)

Pressure (KPa)

Koetoi F. 65.48 31.74 31.74 680.89 Koetoi F. 97.22 10.75 10.75 996.56 Koetoi F.

/ Wakkanai F. 107.97 69.01 69.01 1103.02 Wakkanai F. 176.98 29.75 29.75 1784.73 Wakkanai F. 206.73 73.74 73.74 2077.55 Wakkanai F. 280.43 10.72 10.72 2734.68 Wakkanai F. 291.15 100.48 100.48 2938.81 Wakkanai F. 391.64 49.5 49.5 3944.4 Wakkanai F. 441.13 32.33 32.33 4433.35

134

Table 4-4 lists the measured total mineral concentrations of the liquid water and depth

data, as well as calculated density factors used in the simulation to modify liquid

densities. The porosity and permeability data for the well models are listed in Table 4-5.

Note that permeability values in Table 4-5 are converted using a constant liquid density

of 1000.1 (kg/m3) and a constant viscosity of 0.0011 (Pa•s).

A series of simulations for the two 1-D models are conducted as follows:

1. Simulations using hydrostatic conditions (i.e., no vertical flow conditions) are

first carried out for determining top and bottom boundary conditions, in terms of

pressures and temperatures, by matching the measured temperature and pressure

data of Table 4-2 and Table 4-3. The results of the steady-state simulations are

then used as base models for flow-scenario analysis.

2. Water is injected from the top or bottom boundaries of the two well models, with

different injection rates. This is to estimate downward flow or upward flow

conditions, while keeping constant temperature data on both top and bottom

boundaries.

135

Table 4-4 Measured total mineral concentrations and calculated density factors used for modifying water density as a function of total concentrations at depth

Well Depth(m)

Total concentration(ppm)

Density factor

HDB-7 41.61 4,698 1.003 HDB-7 131.13 9,928 1.008 HDB-7 320.52 29,107 1.027 HDB-7 343.42 31,572 1.030 HDB-7 398.90 28,850 1.027 HDB-7 442.20 31,094 1.029 HDB-7 496.25 28,440 1.026 HDB-7 506.64 29,230 1.027

- HDB-8 63.15 3,255 1.001 HDB-8 101.87 4,204 1.002 HDB-8 283.82 11,469 1.009 HDB-8 344.45 12,395 1.010 HDB-8 400.79 12,014 1.010 HDB-8 449.14 13,066 1.011 HDB-8 465.34 15,175 1.013

Table 4-5 Permeability and porosity values for different geologic units/layers

Geologic Unit Hydraulic conductivity(m/sec) Permeability (m2) Porosity

Quaternery Sediments 1.00E-06 1.10E-13 0.55

Yuuchi F. 5.70E-10 6.27E-17 0.55

Koetoi F. 3.59E-09 3.95E-16 0.55

Wakkanai F. 1.36E-08 1.50E-15 0.45

Masuhoro F. 5.00E-10 5.50E-17 0.4

Cretaceous rock 1.00E-11 1.10E-18 0.4

Fault 1.00E-10 1.10E-17 0.55

In the literature (e.g., Wu et al., 2004; 2007a), geothermal gradients or

temperature profiles are very sensitive to and thus useful in estimating water percolation

fluxes in the subsurface. Here, we use the measured temperature data to estimate both

flow rate and flow directions at the two wells. Figure 4-5 through Figure 4-7 show

comparisons between simulated temperature/pressures and measurements for Well HDB-

136

7. Figure 4-5 shows that at Well HDB-7, if we inject water from the bottom, i.e., water

flows up, the simulated temperature data cannot match the measured values. However, if

we inject water from the top, Figure 4-6 indicates the measured temperature could be

fitted well at a flow rate of 3 mm/yr. Figure 4-7 presents pressure profiles simulated with

water injection at the top and indicates that the lower injection rates match measured

pressure better. The difference between simulated and measured results in Figure 4-7

results primarily from the 1-D flow assumption in the model, while under field conditions,

downwards flow will be subjected to multidimensional flow, find least flow resistance

paths, and the top pressure would be lower.

Figure 4-5 Comparison of simulated and measured temperatures at Well HDB-7 with water injected at bottom boundary at a rate of 0, 1 mm/yr, 10 mm/yr, and 50 mm/yr.

Simulated Tempe rature Prof ile at HDB-7 w ith Up Flow

0

100

200

300

400

500

600

700

0 10 20 30 40

Temperature (oC)

Dep

th (m

)

Observation

No Flow

B1 mm/yr

B10 mm/y r

B50 mm/y r

137

Figure 4-6 Comparison of simulated and measured temperatures at Well HDB-7 with water injected at top boundary at a rate of 0, 3 mm/yr, 10 mm/yr, and 50 mm/yr.

Figure 4-7 Comparison of simulated and measured pressures at Well HDB-7 with water injected at top boundary at a rate of 0, 3 mm/yr, and 10 mm/yr.

Simulate d Tem perature Profile at HDB-7 with Dow n Flow

0

100

200

300

400

500

600

700

0 10 20 30 40

Temperatur e (oC)

Dep

th (m

)

Observation

No Flow

T3 mm/yr

T10 mm/yr

T50 mm/yr

Simulated Pressure Profile at HDB-7 with Down Flow

0

100

200

300

400

500

600

700

0 2000 4000 6000 8000

Pre ssur e (kPa)

Dep

th (m

)

Observation

No Flow

T 3 mm/yr

T10 mm/yr

138

Figure 4-8 through Figure 4-10 show the results for Well HDB-8. For this well,

Figure 4-8 shows that the best estimated flow rate is at about 6 mm/yr, injected from the

bottom boundary. This means upward flow at this location. If injected from the top,

Figure 4-9 shows that simulated results cannot match the measured temperature profiles

at all. Figure 4-10 shows the results and comparisons of pressures for upward flow,

indicating a good agreement for a flow rate of 6 mm/yr.

Figure 4-8 Comparison of simulated and measured temperatures at Well HDB-8 with water

injected at bottom boundary at a rate of 0, 6 mm/yr, 10 mm/yr, and 50 mm/yr.

Simulated Temperature Profile at HDB-8 w ith Up Flow

0

100

200

300

400

500

600

700

0 10 20 30 40

Te mper ature (oC)

Dep

th (m

)

No Flow

B6 mm/yr

B10 mm/y r

B50 mm/y r

Observation

139

Figure 4-9 Comparison of simulated and measured temperatures at Well HDB-8 with water

injected at top boundary at a rate of 0, 10 mm/yr, and 50 mm/yr.

Figure 4-10 Comparison of simulated and measured pressures at Well HDB-8 with water

injected at bottom boundary at a rate of 0, 6 mm/yr, 10 mm/yr, and 50 mm/yr.

Simulated Tempe ratur e Profile at HDB-8 w ith Down Flow

0

100

200

300

400

500

600

700

0 10 20 30 40

Tem perature ( oC)D

epth

(m)

Observation

No Flow

T10 mm/yr

T 50 mm.yr

Simulated Press ure Profile at HDB-8 w ith Up Flow

0

100

200

300

400

500

600

700

0 2000 4000 6000 8000

Press ure (kPa)

Dep

th (m

)

Observation

No Flow

B 6 mm/yr

B 10 mm/yr

B 50 mm/yr

140

Note that the two wells, HDB-7 and HDB-8, are close to each other, with HDB 8

further inland. A comparison between simulation results and the measurements of

temperature and pressures, discussed above, indicates different flow directions, i.e., flow

at HDB-8 is upwards (discharge) and flow at HDB-7 is downwards (recharge). This

indicates that there likely exists a fault separating the two wells, and the fault behaves as

a closed boundary or low-permeability barrier to flow between the two wells.

4.8 Summary

A physically based conceptual and numerical model is presented for simulating

fluid and heat flow and solute transport through fractured fault zones using a multiple-

continuum medium approach. The suggested multiple-continuum concept is a natural

extension of the classic double-porosity model, with the fracture continuum responsible

for conducting global flow, while vuggy (if any) and matrix continua, locally connected

and interacting with globally connecting fractures, provide storage space for fluid and

solute.

The proposed conceptual model can be implemented into a general

multidimensional numerical reservoir simulator TOUGH2 using a control-volume, finite-

difference approach, which can be used to simulate single-phase flow, solute transport,

and heat transfer in 1-D, 2-D, and 3-D fractured reservoirs. Model application is

demonstrated by modeling two well-flow problems at ambient geothermal and water flow

conditions. Data from the Horonobe URL site were analyzed to demonstrate the proposed

approach and to examine the flow direction and magnitude on both sides of a suspected

fault.

4.9 Recommendations for Future Work

We recommend the following future work as a natural extension of the present

study.

1. Develop a general-purpose reservoir simulator for single-phase water flow,

density-dependent solute transport, and heat transfer, based on the TOUGH2 code.

2. Investigate flow characteristics in typical fault zones with fractured vuggy rock.

3. Incorporate the effects of rock deformation on flow calculation.

141

4. Perform site-specific characterization modeling studies.

4.10 References

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24(5), 852–864, 1960.

Biot, M. A., General theory of three dimensional consolidation, J. Appl Phys., 12:57-64,

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Bird, R. B., W. E. Steward, E. N. Lightfoot. Transport Phenomena, John Willey & Sons,

Inc., New York, London, Sydney, 1960.

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Forsyth, P. A., Three-dimensional modeling of steam flush for DNAPL site remediation,

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Kazemi, H., Pressure Transient Analysis of Naturally Fractured Reservoirs with Uniform

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analyzing fluid flow in porous media, Water Resources Research, 12(1), 57-64, 1976.

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145

5 概要調査地域に於ける断層の調査法に関して

5.1 調査のストラテジー

概要調査においておそらく最も重要な調査対象の１つは断層である。断層は周辺の水

理場を支配すると言っても過言ではない。従って、処分場の長期性能評価や安全性から

鑑みると断層の水理性状を正確に把握する必要がある。処分場予定地の近くに大きな活

断層が存在すれば、場合によっては不適格要因にもなり得る。さらに、対象とする地層

に断層を避けて処分場のパネルが余裕を持って配置できるだけのボリュームが存在する

事を概要調査の早い段階で確認する必要がある。これらの事から、概要調査段階におい

て断層の性状の特定は最優先課題と言える。本章では断層の地質および水理性状の調査

を正確に効率良く行う為の調査ステップについて述べる。

断層は物性に関して大きな不連続面を形成する。母岩と比較して力学的、地質学的な

物性の違いもさることながら、水理学的な物性の対比が極端に大きい場合が多々ある。

断層は一般にコア部とそれに付随したダメージゾーンに分けられるが、コア部の厚さや

ダメージゾーンのダメージの度合いとヒーリング、再活動の歴史によって全体の水理学

的特性が異なる。

断層に関する情報としては、位置、幅、走行、傾斜などの幾何学的データを始め、タ

イプ、過去に動いた年代、ずれ幅、岩相や地質等の情報がある。これらの情報はボアホ

ールを掘削する前にリモートセンシング、地表地質調査や物理探査手法を使ってある程

度収集が可能である。また、第 2 章と第 3 章に述べたように、これらの情報と断層の水

理性状に相関性が見られた事例があったり、文献によっては相関付ける手法を提示して

いる。断層が数多く存在する場合、すべての断層を侵襲的に調査するには莫大な費用が

掛かるばかりでなく、ボアホールが流れの場を乱したり、将来の早い水みちになる可能

性がある。専門家によって意見の分かれるところではあるが、断層の長さ、ずれ幅、ダ

メージゾーンおよびコア部の幅の相互の線形的相似スケール則を使うのも有用である。

従って、概要調査では主要と考えられる断層は直接水理試験を行う必要があるが、その

他の断層に関しては、地質情報などの断層パラメーターと水理性状の相関性を仮定して

予め水理性状の予測を行えば、ボアホールを使った効率的な水理試験の計画策定の手掛

146

かりとなる。しかしながら、主要断層の水理性状の正確な把握には直接的な水理試験は

欠かせない。

次のセクションでは概要調査段階における断層の調査ステップについて述べる。

5.2 断層調査のステップ

5.2.1 文献調査

対象とする地域において断層に関する既存の文献調査を行う。断層の位置、性状、地

質、さらに水理、地化学特性に関する記述があればデータを抽出しデータベースに加え

る。この時点で地質情報、形状データなどの情報があれば断層の概念モデルの構築を開

始する。ダメージゾーンの大きさ、偏り、地質等の断層に関するパラメーターから大ま

かな水理性状を予測しておくことが望ましい。

我が国では如何なる概要調査地区であっても複数の断層が存在すると予期されるが、

概要調査段階ですべての断層の性状を把握する必要は無い。大まかな地下水の流動方向

が予測可能であれば、最優先的に調査すべき断層は候補地点から下流側の大きな断層で

あると考える。

5.2.2 衛星、空中測量によるリニアメントの抽出

衛星写真や空中写真を使ってリニアメントを抽出する。重力データに関しては、近年

GRACE と呼ばれる 2 個の衛星を使って重力のシグナルの差を読み取る事によって従来

の精度より 1 オーダー高い精度の重力データを得ることが可能になった。対象とする地

域をカバーするデータが入手可能であれば、前もって衛星による重力データ解析を行う

事を勧める。加えて、セクション 3.3 で述べた ALSM (Airborne Laser Swath Mapping) 又

は LIDAR(Light Detection and Ranging)と呼ばれるレーザー測量技術を使った空中測量を

行って正確な地表面のデジタルマップからリニアメントを判読する事を強く勧める。

LIDAR は近年米国で断層のトレースの発見に極めて有効であるとされる技術である。我

が国においても断層の痕跡を確認した成功例がある。LIDAR は空中磁気や重力、電磁波

などの測量と同時に行える可能性もある。

147

5.2.3 地上物理探査

リモートセンシングや空中探査で発見されない断層及び地質構造を特定する目的で 3

次元の地震波探査を行うことを提言する。3 次元の地震探査は非侵襲的に地下の内部構

造を 3 次元的に推定できる唯一の手法である。一般に地震探査は結晶質岩より堆積岩の

方がより良い成果が得られているが、対象とする地層がどの岩種であっても、3 次元の

震探を行うべきである。3 次元の震探データから断層を避けながら必要な大きさの処分

場パネルが建設できる十分な余裕のボリュームが対象深度の地層に存在するかどうかの

手掛かりが得られるはずである。3 次元地震波探査に加えて、AMT（Audio frequency

Magneto Telluric）法や SP（Self Potential）等の地表物理探査を行う。AMT は主として

地層の空隙率に対比がある際に有効で、SP 法は水の流動を捉える。従って、断層その

ものあるいは断層と母岩の空隙率に変化がなく、地下水の流れが極めて遅い場合はどち

らの手法を使っても断層を特定できない可能性がある。しかし、AMT や SP で検知でき

ない断層は水理的にはさほど重要でないと言える。

5.2.4 地表地質調査とトレンチング

上記と平行して、あるいは上記の結果から、断層が存在すると考えられる地点での地

表地質調査を行う。地質分布や断層の位置、性状などの推定を行う。既存の文献、ある

いはリニアメントの調査から主要と思われる断層が地表、あるいは地表近くに存在すれ

ば断層と直交する方向にトレンチを掘り、断層の内部構造を把握する。

断層が普遍的に存在する日本列島の地質環境を鑑みれば、重要な課題の１つは、対象

とする断層が活断層であるか、あるいは長期に渡って不活断層であるかを見極める事で

ある。地表踏査ならびにトレンチングによって断層周辺の岩質を調査し、最近では何年

前に動いたのかを調べる事によって鉱脈や変成などの岩-水の相互作用の程度が推定で

きる。次に現在の応力場を知る事によって地殻の変形および構造レジームに関する知見

が得られる。

種々の地質パラメータや断層形状を計測し、第 2 章で触れたスケール則が当てはまる

か検討する。また、断層部のみならず、オーバーステップの地質性状の調査を行う。

148

5.2.5 ボアホールによる調査

地質調査や物理探査から得られた情報を元にボアホールを掘削する適切な位置を決定

する。概要調査段階では断層性状の把握の目的のみの為に本数の限られるボアホールを

掘削するのは一考の余地があると考えられがちである。しかしながら、我が国でのこれ

までの事例から考えると、（１）結晶岩においては、1 千メートル級のボアホールを掘

削すれば大小なりとも必ず断層を貫通する。（２）岩種に拘らず、断層は周辺の水理場

を大きく支配する。以上の事から、敢えて断層を避けて掘削する必要は無いし、おそら

く避ける事は困難であろう。しかし、幌延の大曲断層の事例のように、断層のダメージ

ゾーンにおいて、場所によっては高い透水性があると考えられ、母岩そのものの透水性

が低い場合は、断層を狙ってボーリング孔を打たなければ、大きなスケールでの主要な

水理の特性が解明できない恐れがある。如何に少ない数（1~2 本）のボアホールで如何

なる試験を行い、如何なる解析手法を用いて最も効率的で正確に断層の水理性状を把握

するかが当面の課題でなる。

リニアメント、物理探査、地表地質調査から複数の断層が存在すると推定される場合

は、処分場の位置より下流側に存在する断層の中で最も主要と考えられる断層を特定し、

その断層を貫通、もしくは上流側のダメージゾーン内にボアホールを掘削するのが最も

効率的である。断層周辺の詳しい水理場の特定の為には断層の両側に複数の観測点が必

要となる。概要調査段階で掘削するボーリング孔の数を最小限に抑える為には、JAEA

の正馬様用地の MIU-2 孔のように断層を貫いて掘削するのが望ましい。断層がほぼ垂直

であれば、傾斜掘りも考慮に入れるべきであるが、傾斜掘りはコスト嵩むうえに、モニ

タリング機器の設置においても垂直孔よりは課題が多い。

次に決定しなければならない問題は全孔コアボーリングをするか否かである。諸外国

の処分プログラムに於いてはコアボーリングが日常的に行われているが、我が国の概要

調査段階に限って言えばコストパフォーマンスは低い。従って、予算が極めて限られて

いれば部分的なコアの採取で良いと考える。ただし、その候補地が精査段階に進む可能

性が高ければ、全孔コアを採取する価値がある。無論、掘削速度、逸水などの掘削ログ

や堀屑のサンプリングは当然行うべきである。また、既に常套になっているが、掘削水

に SF6 等のトレーサーを混ぜ入れる事を勧める。断層を貫通して掘削する場合、可能で

あれば断層を貫通した時点で新たなトレーサーの混入が望ましい。

149

掘削が完了次第、あるいは必要であれば掘削の段階ごとに検層を行う。スタンダード

なワイヤーライン検層に加えて、比較的透水係数の低い孔では、孔内水を脱イオン水で

置換して測定を行う安価な FEC(Fluid Electric Conductivity) ログを勧める。この手法を使

えばボアホールに沿って流入点の特定および、それらの相対的な透水係数の推定が可能

である。掘削前や掘削中に断層の位置が特定できない場合は掘削後にボアホール内の物

理探査（VSP、リバース VSP 等）を行った事例もあるが、概要調査段階では基本的には

不要と考える。

断層の水理性状の推定に最も確度の高い方法は長期揚水試験、続いて長期圧力回復試

験を行うことである。その際、表層の堆積層や多区間で揚水試験を行う必要は無い。基

本的には基盤岩全体を 1 区間で揚水試験を行ってもよいが、ボーリング孔が断層を貫い

ている場合は、断層の上側と下側をパッカーで区切って、それぞれ揚水試験を行う。揚

水をしていない区間は圧力のモニタリングを行う。その際ボアホールが水の短絡路にな

らないようにコア部にパッカーを設置する必要がある。東濃地域の月吉断層の例では上

盤と下盤でおよそ 40m の水頭差が存在しているが、MIU-2 孔のパッカーを除去した際に

水頭の高い下盤側から上盤側に大量の水が流れ込んだ経緯があった。同様に、逸水箇所

等、透水性の高い部分が明確な場合はその区間をパッカーで隔離し揚水し、上下の区間

でモニタリングを行う。

概要調査段階での揚水試験は基本的には単一孔試験であるが、他にボアホールが存在

すれば、モニタリングを行う。この場合も、多区間に区切る必要は無く、断層があれば

それらを区切るだけで良い。その理由は、概要調査の目的は大きなスケールでの断層の

役割と断層の水理パラメーターを把握する事であり、小さなスケールの不均質性の調査

は不要だからである。

揚水試験の際、地下水中の地化学成分を同定する為に揚水した地下水のサンプリング

を行う。掘削水に混ぜたトレーサーの検出、イオン濃度（Na, Cl, HCO3 等）、希ガス、

同位体、溶存有機物、その他の成分の分析を行う。これらのデータを元に地下水の起源

や年代が推定できれば周辺の地下水流動場の評価につながる。

断層に関わる長期揚水試験において課題となるのは、圧力遷移データの解析の手法で

ある。断層はコア部やダメージ部は平面的な広がりを持つが、基本的に 3 次元構造であ

150

り、堆積岩では層序に垂直もしくは大きい傾斜角で交わり、水や石油資源の分野で使用

されている水平放射状流を前提とした井戸試験の解析手法がそのまま当てはまらない恐

れがある。新たな解析手法や数値モデルを使って逆解析を行う必要がある。

5.2.6 長期モニタリング

セクション 3.1 及び 4.7 の事例に見られるようにボアホール沿いの水頭分布や温度分

布を測定することによって地下水の流れの方向や流量が推定できる可能性がある。しか

し、掘削直後の水頭や温度分布は掘削によって場が乱されている恐れがあることから、

掘削後の長期にわたるモニタリングが必要である。また、気候や季節の変動にどのよう

な反応を示すかをモニタリングする事によって断層の水理性状が推定出来た事例もある。

概要調査に数年掛ける予定である事を考えると、モニタリングの装置に関しても 5，6

年の期間にわたって性能が保証されるか、メンテナンスが容易である事が望まれる。一

般に圧力センサーの寿命はさほど長くない。明らかに故障をしなくてもドリフトが起き

て正確なデータが録れなくなる可能性も考えなくてはならない。東濃地域の長期モニタ

リングプログラムの例を見ても、約半数のボアホールが使用不能となっている。重複し

てセンサーや機器を設置して故障の場合に備える事が肝要である。電子的なセンサーの

故障を担保する為にピエゾメーター管を埋設する事も考慮すべきである。

ボーリング孔沿いの温度分布は光ファイバーを使った温度センサーを使用することが

望ましい。光ファイバーセンサーは全長に渡って連続的に温度測定が可能である。ケー

シングを設置する場合はケーシングの外側にファイバーを埋設すれば揚水試験区間や圧

力モニタリング区間、孔内の流動に影響されずに温度分布の連続的モニタリングが可能

となる。

5.3 フローチャート

Figure 5-1 に前セクション 5.2 で述べた調査のステップのフローチャートを示す。

断層の調査は概要調査の一部であるので、概要調査全体のフローに関しては Karasaki ら

（2006）を参考にされたい。

151

文献調査

、リモセンデータリニアメント解析、、重力温度分布 GPS,

、ハイパースペクトル InSAR、ALSM, 航空電磁気測量

ボアホール掘削、掘削逸水ログ

長区間長期揚水試験、圧力地下水サンプル

必要十分な？本数 No

3次元概念モデル

地質調査、マッピングトレンチ

水理地質モデル

データベース

、パッカー圧力・温度センサー設置

？断層

Yes

No

地表物理探査、、3次元地震探査重力

、Resistivity SP, AMT

、中心下流に？断層確認

Yes

No

Yes

Yes

ボアホール掘削位置の決定

ボアホール物理探査VSP, reverseVSP

断層が明確No

挙動予測

性状予測

フィードバック

長期モニタリング、圧力温度

透水係数地化学データ

ログデータ

、検層コアデータ検層・FECログコア採取

断層の概要調査は不要

Figure 5-1 断層調査ステップの流れ

152

5.4 References

JNC, 高レベル放射性廃棄物の地層処分技術に関する知識基盤の構築、－平成 17年取りまとめ－、JNC TN1400 2005-020、2006.

Karasaki, K, J. Apps, C. Dounghty, H. Gwatney, C.T. Onishi, R. Trautz, and C.F. Tsang, Feature Detection, Characterization and Confirmation Methodology, NUMO-LBNL Collaborative Project Report, March 2007.

153

Acknowledgements

Funding for this work was provided by the Nuclear waste Management Organization of Japan, through U.S. DOE Contract No. DE-AC02-05CH11231.

154

Appendix 1

NUMO‐LBNL Collaborative Project

Development of Hydrologic Characterization Technology of Fault Zones

‐Work Plan‐

August 2007

155

1. Background and Objectives

In 2006 ANRE (Agency for Natural Resources and Energy) jointly with JAEA (Japan Atomic Energy Agency) identified outstanding technological issues and needs regarding the research and development for geologic disposal of HLW subsequent to the publication of the Second Progress Report by JNC (Japan Nuclear Cycle Development Institute) in 1999. Research organizations in Japan as well as NUMO have been conducting investigations on these issues and needs. In the area of ground water hydrology, four R&D needs were identified and are currently being investigated: Improvement of groundwater flow characterization technology, development of testing and characterization technology in coastal areas, development of testing equipment and technology, and field application of testing and characterization technology. NUMO has been incorporating the results of the outcome of these R&Ds as they become available and deemed appropriate and is in the process of systematizing the testing and characterization technology to form a solid technical foundation for selecting the sites for detailed investigation.

Meanwhile, NUMO has recently conducted a collaborative study with DOE/LBNL, “Optimization of Site Investigation Program” as part of an effort to evaluate testing and characterization technology from the performance assessment perspective. The study has identified the hydrologic properties of fault zones as one of the most important parameters that need to be evaluated during the preliminary investigation stage. Based on the experiences learned at the Mizunami and Horonobe URLs as well as at numerous mines, dams and tunnels, and given the geologic environment of the Japanese Islands, faults are likely to exist almost ubiquitously, which need to be assessed both at the preliminary and the detailed investigation stage (the length scale of the faults of interest would range from several kilometers in the former down to several hundred meters in the latter investigation stage). However, none of the four R&D activities mentioned above sufficiently addresses the development of systematized hydrologic characterization technology specifically tailored for fault zones. At present, it is necessary to use perhaps overly conservative values for the hydrologic parameters of fault zones for the design and performance assessment of a repository. Therefore, development of a more efficient and reliable fault zone characterization technology is highly desirable.

The geologic properties of faults and the relationship among their geometry, type, fault parameters and internal structures are being investigated mostly in overseas. Hydrologic investigation of faults of various sizes are also being conducted at foreign as well as at domestic characterization sites. However, the relationship between geologic and hydrologic properties of faults is not yet studied sufficiently.

The objectives of the present study is to organize the information available in overseas to ultimately establish an efficient and systematized methodology for hydrologic investigation and characterization of faults at the scale of interest during the preliminary investigation stage for more practical design and performance assessment. The present study will be conducted as a collaborative study between NUMO and USDOE/LBNL. It should be possible to apply/transfer the results of the study obtained at a site in the West Coast of the United States, whose tectonic environment is just as active as that in Japan, to the Japanese repository program when they will be needed, which will be also beneficial to the US program.

2. Activities

This is a project of approximately three-year duration from FY2007 through 2009. In 2007 investigations will focus on a literature survey and feasibility study of the methodology. In 2008 and 2009, fieldwork will be conducted to verify the approach and to systematize the methodology. Below describes the first year activity. Specifics of the second and the third year activities will be developed in FY2007.

156

FY2007

1. Literature Survey

Conduct literature survey and expert solicitation from overseas and at home, to investigate past and current hydrologic characterization activities of fault zones at various scales of interest from the following countries and disciplines.

Countries: USA, Canada, Sweden, Finland, Great Britain, France, Switzerland, Japan, etc.

Dsciplines: Geologic disposal, mineral exploration, civil engineering, earthquake disaster prevention, environmental restoration, etc.

A. Type classification

Classify faults with known hydrologic properties by type based on the geologic properties and summarize the respective hydrologic properties. At the same time, the differences and similarities between the geologic and hydrologic properties of these faults and those in Japan will be highlighted.

B. Field investigation and characterization technology

Collect and organize information regarding actual examples of a sequence of field investigation, modeling and analysis of fault zone hydrology including the technology used. Contrast the differences and similarities with those in Japan. Identify the applicability and the limitations of them to be used at the preliminary investigation stage.

2. Investigation of characterization technology

A. Development of investigation and characterization flow and planning

Examine the most optimum investigation, modeling and analysis methodology for a given fault type classified in Task-1 at the scale of the preliminary investigation stage. Conduct modeling analysis of the most advantageous borehole configurations and testing approach when multiple fault types coexist. Summarize the findings and construct a systematized flow of testing and characterization procedure. In addition, plan and design a field investigation to verify the approach including the siting of a filed demonstration site.

B. Review of results

Conduct critical review of the results obtained in Task1 and deliberate on the verification plan by soliciting participations of experts from domestic universities and research organizations.

FY2008~FY2009

1. Field Investigation

Conduct field investigations at the site according to the plan laid out in the previous year. Analyze available information and conduct surface geologic investigation and geophysical surveys to locate the position of the fault and develop comprehensive understanding of the surrounding geology. Predict the hydrologic properties of the fault zone, followed by the planning of detailed borehole investigations. Conduct field tests, modeling and analysis to develop understanding of the hydrologic properties of the fault.

2. Summary Report

157

Evaluate the effectiveness of the testing and characterization approach based on the results obtained from the field investigations, and summarize the systematized approach. Make recommendations for testing and characterization methodology to be employed at the detailed investigation stage. Conduct comprehensive evaluation of the results and discuss issues for the next phase by soliciting participations of experts in hydrology from domestic universities and research organizations.

3. Responsibilities

Activities in each task will be conducted by LBNL. Planning and evaluation of results will be jointly conducted by NUMO and LBNL.

4. Schedule and deliverables.

The three-year schedule is shown in Table 1. A progress report summarizing the results of FY2007 activities will be submitted to NUMO by the end of February 2008. FY2008 and FY2009 deliverables will be defined in separate planning documents.

Table 1 FY2007 FY2008 FY2009 Literature Survey Efficient testing and characterization technology

Field investigation

Reporting

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Geologic and geophysical survey Borehole testing Workshop

Workshop

Development of Hydrologic Characterization Technology of ...

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